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Когда мне будет 20 лет...[/quote]

Bah, I thought it was plural cos it was 20 years. Sometimes I am too logical for my own good.

:P

So would it be Мне было 20 лет in the past?

You use the perfective, becuase it is actually a future event.

In English we say things like

"When I am 20, I will live in Spain"

when what we are actually saying is "When I will be 20, I...