Read this article on the BBC www.bbc.co.uk/news/health-23223751
My DS has always had a bedtime of 7pm, only recently moved to 7.30 (he's now 7), partly because he's an impossible child the next day if he's been up later. Even if he's had a late night he'll never sleep in.
DD (7 weeks) seems to prefer later nights naturally. We've been trying to instill an 8pm bedtime, but it's always at least 9 before she sleeps. I was going by 'what suits each individual child', but now I'm wondering...