Both my daughters are summer born. The main disadvantages from their point of view seem to be being short and having to wait a long time for their birthday parties to come around. DD1, in particular, is flying high academically. DD2 is a bit young to tell, but is in a literacy group with children two years older than her, so I'm guessing she's doing OK
.
I was one of the youngest in my year group in Scotland. I didn't turn 18 until the November of my first term at University. Celebrated my 18th birthday in the bar of my Oxbridge college, and made the barman rather flustered when it dawned on him that I'd been under age for the previous month!
But it's all anecdotal. There doesn't appear to be much doubt that being young in the year can be a big disadvantage, particularly for children that are less academically-minded and less ready to start formal learning.