I think there is fairly solid evidence that the use of cannabis at a young age ie before the age of 18, has been shown to produce some cognitive impairment when put next to those who start using cannabis aged over 18 and the most identifiable deficit seems to be in a measure called the verbal IQ.

*Sigh* Sources...please?

RESULTS: The 69 early-onset users (who began smoking before age 17) differed significantly from both the 53 late-onset users (who began smoking at age 17 or later) and from the 87 controls on several measures, most notably verbal IQ (VIQ). Few differences were found between late-onset users and controls on the test battery. However, when we adjusted for VIQ, virtually all differences between early-onset users and controls on test measures ceased to be significant. CONCLUSIONS: Early-onset cannabis users exhibit poorer cognitive performance than late-onset users or control subjects, especially in VIQ, but the cause of this difference cannot be determined from our data. The difference may reflect (1). innate differences between groups in cognitive ability, antedating first cannabis use; (2). an actual neurotoxic effect of cannabis on the developing brain; or (3). poorer learning of conventional cognitive skills by young cannabis users who have eschewed academics and diverged from the mainstream culture.

Have a look at this link; I'm keen to hear your thoughts on this.

http://www.sciencedaily....2008/10/081014111156.htm

In general, MJ-users reported
marginally higher scores on the BDI [MJ-users
4.6 ± 7.0, range 0–20; controls 1.3 ± 2.0, range 0–6;
F(1,31) ¼ 3.4, p ¼ .08] and significantly greater
scores on the HAM-D [MJ-users 4.0 ± 5.9, range 0–
21; controls 1.0 ± 2.0, range 0–8; F(1,31) ¼ 4.2, p ¼
.05]. Compared to published norms (Beck et al.,
1988; Bennet et al., 1997), 0% of controls and 19% of
MJ-users were clinically elevated (>13) on the BDI.
On the HAM-D, 6% of controls and 13% of MJ-users
evidenced mild symptoms (scores 7–17) and 6% of
MJ-users reported moderate (>18 ) depressive
symptoms (Nixon et al., 2001).
Bivariate relationships
Table 1 shows correlations between the demographic
variables, brain volumes, and depressive measures
for each group. Effect sizes (Cohen, 1988 ) reflecting
the magnitude of the difference between correlation
coefficients (with equal sample sizes) revealed that
the correlations between white matter volume and
BDI scores (q ¼ .89, large effect size) and HAM-D
scores (q ¼ .22, small effect size) were stronger
among the MJ-users compared to controls. All other
effect sizes were not significant (q < .10).