Being in scotland, ur legal sex limit is 16, so you get some people who will go as low as that. Personally, I won't go near anyone under 18, but preferably 21+, simple due to the legal drinking age being 18, but usually (not always) the 21 year olds are more mature. I'm 24.
As an upper limit, I personally don't go more than 4 or 5 years above me, however my sister once dated a guy who was 32 while she was 19 and my mum is currently engaged to someone 14 years older than her.
As for having nothing in common, that's just simply not true. I could if i wanted to, date someone in their mid-30's and have loads to talk about, music being a perfect example. I still prefer 80s/90s music to most the crap that comes out these days.
All in all, age should only matter to you. People will judge, but that's their issue not yours.