Why do Christians become Christians?

[Mister Agenda]

I've seen numbers in the US indicate that ~85% become Christians between 4 and 14. If we use that number for the rest of the world, that would mean approximately 325,500,000 (out of 2.17 billion) became Christians later than 15 years old. 

That's 15% of 2.17 billion. Not very impressive, in my opinion.