Are congenital cataracts in teens rare?

Yes. Congenital cataracts means that you are born with them. They can range from insignificant to severe. Now that rubella (german measles) is pretty rare these days thanks to the vaccine., one does not often encounter that type of congenital cataract any more.
Uncommon. Congenital cataracts are uncommon at birth and most do not affect vision so they would not be noted. If vision is affected, they would have been noticed at an early age. If a teen has an eye exam for another problem, and has a congenital cataract (s) that has not affected the vision, then that would be the age first noticed. Probably not anything to worry about.