Yes that was the same with us. He would love to stay up late with us and we loved it too because we all went for wonderful summer walks. What I noticed around 4.5 mths or so was that he was getting sleepier earlier. When I made the bedtime earlier initially it didn't work at all. But gradually it was better. Then my MCHN suggested even earlier -- 7-730 sleeptime (after a routine). That's what I'm up to now ...

You may find it changes or not with yours. If you love it that way, don't worry -- go with the flow! But if you want an earlier bedtime there are some good books that have gentle ideas on that.