Check age on registration...then assign usergroup

[Staxed]

Ok, I can't search for 'age' on here...so I can't seem to find anything like this..

what i need a few things...

when a user puts in their birthdate on registration...once they hit that submit button...they can no longer change their age...they must ask an admin to change it...

next, base on the age...i need them put into a usergroup (there are two...one for 16+ one for 18+)

I would also need it to update (via cron?) when the user turns one of these ages...

i.e...if the user is 15, on their birthday they should be added to the 16+ usergroup...

these usergroups should be assigned as secondary usergroups...(so you don't have to remove the 16+ to add them to the 18+...but you can if you want)...