hmmm...this is how my experience went...
Parker Kenpo - 2 years, age 10-12
TKD - 2 years, 3 different teachers, each of whom made me start over because they didn't like the other guys, age 12-14
Parker Kenpo - 3 years, Mr. David Brock, age 14-17
TKD/Aikido - 3 years, age 17-20 (college classes, again, made me start over at white belt, not that i remembered much TKD by this time)
Parker Kenpo, Mr Brock, age 20-21 (then Mr. brock closed his studio and I couldn't find somewhere I wanted to train that was near where I was living, so I took 2 years off until he reopened) then, at age 23, I started training with mr. brock again and still do.