Dynamic content in text_field

Hi people

I have a problem. Maybe this is something easy, but I can’t figured
out with my knowledge.

To explain my problem, let’s supposse I have a model call User, the DB
table for users

Users
-id
-username
-age

and a model Account, where a user has many accounts and an account
belongs to user. The DB table for accounts

Account
-id
-user_id
-account_name

in my form for accounts I have a collection_select, to select a user.
Then I want to have a text field for the age of the user. The idea is
that when I choose a user in the collection_select, the
text_field :age get filled automatically with the age of the user
previously selected.

Which is the best approach to figured this out??

I hope you can help me, thanks

Which is the best approach to figured this out??

Spend a year learning javascript?

http://api.rubyonrails.org/classes/ActionView/Helpers/PrototypeHelper.html