I'm 35. I am an only child and that was unusual in my primary/secondary school but I met (and am still friends with) several other onlys from sixth form onwards.
I have two children. My mum is one of two and my dad was one of three. My DH is one of two. My mum didn't have me until later (35 but that was much less common to have first child that late in 70s) and thought it was a bit late to have any more.
If I couldn't have had any more after DD1 then we'd have been really happy with one, but as we liked her so much and there was time for another we were lucky to have DD2.
When I was little I was quite antisocial with other kids and preferred adults...not sure if that's down to me being an only child or just personality. Other than that, I wouldn't say I was spoilt, my parents didn't have a very large income (postman and secretary) but there being only me things weren't spread quite as thinly as they may have been otherwise. Also I remember being glad when I saw my friends' toys and possessions got broken by younger siblings that I didn't have that problem at home.
I am quite gregarious and not shy now. I don't think it affects me now except that if my parents become infirm there is no question of shared responsibility as there's just me. I generally like being an only child, though I'm glad my husband has a sister who I get on with and I am an aunty, and am glad DD1 and DD2 have a cousin.
My daughters were very gregarious from the word go...DD2 probably too young to judge but DD1 seems more confident than I was at her age, chats to people - adults and children- with no trace of shyness and makes friends easily. I think this is personality though, but maybe to do with associating with a gang of other kids of various ages from the age of 11 months at childminders - I don't know.