I can't relate to teenage or early 20s characters/actors. I've never liked kiddie companions.
Wow, I take it you've been put off from about 90% of televised Doctor Who, then.
The age of the actors is irrelevant and also remember that at heart this is still a children's show so if they cast younger people in the roles that's perfectly in keeping with the philosophy. Consider yourself lucky they haven't paired the Doctor with a kid (John and Gillian, anyone?).
And I'm now older than Tennant, Eccleston, Davison, Colin Baker and Tom Baker were when they were all cast, and it hasn't put me off!

Alex