I doubt "kids" signing up for AOL are a large reason for the low score. The credit score is from people who wanted to check their score, and I would guess that the number of people <18 who tried to check it is almost negligible.
On the other hand, AOL, Yahoo, and MSN/hotmail all had low scores, which may have something to do with those emails being tied to an instant messaging account.
All we know is that from a sample of 20k scores those with Yahoo email addresses (which could well be 10k or 2k, we don't know) had a lower average than the rest. It says nothing about the whole population of Yahoo/Hotmail/etc. because we don't know how the email addresses were obtained.
It would be very easy to bias this sample by showing ads about finding your credit score to different demographics within each mail provider.
In addition, they don't show variability within the groups. The differences they find among the averages may not mean much in the context of the variations about the averages.
Furthermore, any statistically significant differences may be due to correlation with other factors. Perhaps domain name varies weakly with geographic location-- which the same source claims also predicts average credit scores.
Finally, their chart accentuates the differences by dropping the zero.
On the other hand, AOL, Yahoo, and MSN/hotmail all had low scores, which may have something to do with those emails being tied to an instant messaging account.