Question

Do some people start comics/manga/novels with a scene 10 years before the main time?

Answer 1

I'm not really big on manga and anime, but yes, starting with a flashback is pretty normal, at least in every other media I can think of.

Answer 2

Yes :]

Answer 3

Yes

Answer 4

It often depends, is this a question on whether or not it starts off in the past for a story that you want to write? If so, you can start it whenever.
Thing is, is that starting in the past is a nice way to fill backstory of past events that will be mentioned in the present for the story, it doesn't essentially have to be 10 years ago, it can range from various time periods, the point is, yes, there are shows that do start in the past, not all of them start specifically 10 years ago though.