I have stepkids and a kid of my own, and thus far have covered ages 0 through 10 (kid, stepgrands) and 13-mid-30s (steps).
I'm finding 10 pretty good and would say things really started getting noticeably more fun about 8. But 10 is fabulous! I can issue requests (or instructions) like, "Please empty the dishwasher," and -- he does. It's fabulous. But also there's lots of fun stuff we can do together where he is really a contributing/participating member of the process/discussion/etc. Also, he will sleep past 7 a.m. if allowed to do so (that pretty much didn't start happening until he was about 8).
The teenage years were a mixed bag (steps, though, obviously, we were just embarking on our relationship then and it's probably not an entirely fair comparison to a kid one has known from day 1 and who's not trying to figure out, as I was, how stepparents/kids relate). But from the early- to mid-20s they've been pretty much fun, too. :) .
The grands are reminding me why really the early years are not my favorite.
Though don't get me wrong, there have been wonderful events, moments, activities at every age. But I definitely prefer moderately independent, able to follow simple instructions, and not-too-likely-to-engage-in-self-destructive-behavior.
On a positive note if (like me) you turn out to be someone who prefers older kids to little kids, then the trend will be in a good direction. It seems to me it would be really difficult to be a parent if your most preferred ages were the really young ones, because it would all be a downhill slide after the early years.
If I could do one thing different vis-a-vis my kid's earlier years, it would be to make more use of paid childcare. We are fortunate to have lots of good choices available around us, and I wish we'd relied on them more -- particularly as a sociable only child, he loved the time he did have in daycare (roughly 16 hours/week from 2 months until he was 4 and then 24 hours for the final year before kindergarten, but throw in an extra 8-16 hours of grandma care as well) and would, I think, have benefited from getting more of it, as would I have from doing less juggling and having more energy for the time we did spend together.