"in 15 years" vs "in 15 years' time". They are about the same, but the latter emphasizes the time span by adding one more word. Sometimes we want to draw attention to the length of time more than what "in 15 years" may afford. So we say "in 15 years' time", "in the span of 15 years", etc. to highlight it. That's all.

Not quite the same. They can both mean "fifteen years from now", e.g. "I will retire in fifteen years". But if you say something like "I haven't been on vacation in fifteen years", then you wouldn't use the other form.

Yes, they are the same.

i think they have not the same meaning u should say them in sentence when u use "time"it refers to an special period if u say them in sentence we can realize better

i am not sure but i think u must say:in 15 year?(not years)time