Users under 18 automatically sent to usergroup

[Neo_obs]

We are implementing age groups for people under 18 they get different features and we are hoping this solves some of our problems. What I am asking for is a mod that will automatically place a user in a certain usergroup if they are under 18 and when they turn 18 automatically place them into the normal field.

I know people can change their age this is not a big concern we just want to have this.

right now we are using 3.5.0 and have no plans to upgrade to 3.6 untill we can update our other server settings

we use php 4.3.1
and MySQL 3.23.58
with apache 2.0.5

al help is greatly appreciated thank you.