I've read quite a lot that with bipolar you're more likely to have depression set in earlier on in life than people with uni-polar depression.
Mania/hypomania doesn't usually set in until later though.
OP: I'd try not to worry about it too much. You'll almost certainly noticed (at least in hindsight!) if you've began experiencing depression and/or (hypo)mania. If you have some close relationships those people will likely notice and point things out too. 
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