There is no age.
I know a couple that claimed to "fall in love" with each other when they were 5 & 7, and now, 40 years later, they are still together.
Its not something you decide either, its something that takes over you, you just know when your in love.
I don't think 11 & 12 yr olds should "fall in love" just because the fact that they don't comprehend love yet, but it can happen :]
u can't decide when to fall in love...it just happens