Age and the rate of offending

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The preceding sections of the report explored the development with age in the two birth cohorts of participation in recorded delinquency. Participation in, or the prevalence of, delinquency, was found to increase with age to a peak of 5.8% at 16 years. One in every seventeen persons born in 1987 was identified by police as having committed an offence at the age of 16 (Figure 3).

Another aspect of delinquency which varies with age is the number of offences which each offender commits at a given age: that is, the age-specific rate of offending, or age-specific incidence. If, on average, teenage offenders commit more crimes per offender per year than children or adults, then the total number of crimes by teenagers will also be higher, even without an increase in the number, or prevalence, of offenders. Thus, the age-specific volume of crime, or crime rate, which is the phenomenon charted in the age-crime curve (Figure 1), is the product of age-specific prevalence and age-specific rate of offending, or incidence. This section examines changes with age in the rate of offending.

Figure 17 shows the mean number of recorded incidents per active offender, by year of age.¹ The incidence of recorded offending changes little from 5 to 8 years of age, then increases from an average of 1.1 incidents per 8 year old offender to 1.8 at 15 years. After that age, it begins to decrease. This is true of both boys and girls (Figure 18), although the increase with age is much greater for boys: up to 8 years old, male offenders are implicated in an average of about 1.1 incidents per year, and the incidence for girls fluctuates, but is a little higher. For boys, age-specific incidence then rises by 77% to an average of 1.95 incidents at 15 years old; whereas for girls, it rises by only 22% from 1.18 incidents at 8 years old to 1.44 at 15. Since there are 2 to 4 times as many male as female offenders in this age range (Figure 2), and their increase with age in rate of offending is much greater (Figure 18), it is they who make by far the greatest contribution to the observed overall increase after the age of 8 in age-specific incidence.

Figure 19 shows the mean number of incidents per active offender by year of age, disaggregated into the three types of offences. Only the most serious recorded offence in each incident is considered.² There are some very interesting aspects to Figure 19. The rate per offender of property offending is by far the highest, but it does not increase with age like the overall incidence of offending (see Figure 17). However, there is a short-term increase from 12 to 15 years, which contributes to the increase between 12 and 15 in the overall incidence rate. The incidence of offences against the person increases from the age of 5 to 12, after which it becomes almost perfectly constant. The increase with age in the incidence of "other" offences is the most dramatic: it increases by a factor of 10 from 0.06 at 11 years to 0.62 at 17, and has not levelled off by the age of 17, although the rate of increase has decreased slightly. Thus, there are different explanations for the changes in different segments of the age-incidence curve shown in Figure 17. From 8 to 11 years, it is largely (about 80%) due to the increase in the rate of offences against the person. From 11 to 12 years, person and "other" offences each account for about half of the increase in the overall rate of offending. From 12 to 15 years, it is the increase in the rate of "other" offences, from 0.14 to 0.49 per offender, which accounts for most of the increase in overall incidence, with the remainder being due to property offending. The decrease after the age of 15 in the overall rate of offending is entirely due to the rapid decrease in property offending, as the rate of person offending is constant, and that of "other" offending is increasing rapidly.

The importance of "other" offences in the overall increase in incidence after 11 years of age can be seen clearly in Figure 20, in which offences against the person and property are combined. Although the annual average rate of person and property offending increases slowly over the entire age range from 6 to 15 years, and increases by a total of 0.11 incidents from 11 to 15 years, the rate of other offending increases by 0.43 incidents over the same 4 year period – although its base rate is far lower. Figure 21 shows that this proportionately huge increase after 11 years old in the rate of other offending is mainly due to offences against the administration of justice³ and drug offences, the incidence of both of which increases very steeply.

There are also some interesting differences between boys and girls in the breakdown of incidence by type of offence. The pattern for boys (Figure 22) is, not surprisingly, very similar to the overall pattern (Figure 20). For girls (Figure 23), there is no increase at all with age in the incidence of offences against the person or property: it fluctuates around 1.1 incidents per offender, and decreases slightly after the age of 12. But the incidence of "other" offending rises sharply after 11 years of age, and does not level off or decrease at the end of the age range. Approximately 75% of the increase from 11 to 15 years of age in the rate of "other" offending by girls is due to offences against the administration of justice, with the rest being due to drug offences (Figure 24).

In summary, the rate, or incidence, of recorded offending fluctuates around 1.1 incidents per active offender per year from 5 to 8 years of age, then increases to a maximum of 1.8 recorded incidents at 15, and decreases thereafter. The increase in incidence is much greater among boys than girls. From 8 to 11 years of age, the overall increase is largely due to an increase in the rate of offences against the person by boys. From 12 to 15 years, the increase is largely due to an increase in the rate of administrative and drug offences by both sexes, although boys play a larger role.

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Note

1. The minimum value on the y-axis is 1, since, by definition, an active offender is involved in at least one incident. The numbers of active offenders at each age, on which the mean incidences are based, are shown in Table A.2 in the Appendix. For the age range (8 to 14 years) where there are data for both cohorts, the age-specific incidence statistics are similar enough to each other that they can be treated as one population, except for the probable underestimate for 14 year olds born in 1990 (see note 1 in "The age-crime curve"). Therefore, reported values for the age range 8-14 are the mean of the values for the two cohorts. Two-tailed t-values for the differences between the cohorts in incidence at ages 8, 9, ..., 14 are: -1.48, 0.29, -1.99, 1.76, 0.27, 2.09, 4.25. These statistics should be interpreted with caution, since these are not normally distributed random samples. Only the t-values for 10, 13 and 14 year olds are significant at p<.05 (p₁0=.047, p₁₃=.037, p₁₄<.0001).
2. The minimum value on the y-axis is 0, since an offender who is active at a given age may not be implicated in at least one incident of each type at that age. Indeed, few are, and the means are practically all below 1.
3. The most common offences against the administration of justice are violations of bail or probation conditions, and failure to appear for court. Other, much less common administrative offences include prison breach, escaping from custody, and other very infrequent offences (Appendix Table A.1).