To change the age in google+(plus)

Yes, I've got the same issue! I don't believe Google does this for anything other than legal reasons. I'm 16 and a regular Google user, but I ran into the same problem when I entered my birthdate as I saw no EULA telling me I had to be 18+. It gave me one chance to confirm, saying that my birthday was invalid after I entered the date that makes me under 18. I again entered my real birthdate as I didn't get the hint. No big deal though, being age of majority to use an online service is fairly common. I would have liked to have beta tested Google+ but that's alright, I don't need another distraction. My nice, previously black bar, has turned grey, though, so I have a sinking feeling other Google features will disappear for the next two years!

Don't think that Google is mocking you because of your age. They know they know the value of extremely smart youth like Tesca Fitzgerald (14), who kicked off the Google Science Fair. This is probably mainly a legal issue, and until Google+ gets a EULA that allows minors, I believe the site will be open to 18+ only. I've been beta testing Google services for the past couple years, and I'm good friends with a couple people who make regular trips to the Googleplex, but it really doesn't matter. Google can't do anything if you admit to being under 18. This specific one has to do with the checkbox, I believe. You are required to assume responsibility for anything that happens with your information, and you can't legally be responsible for your own actions until you're 18.

Chances are, anyone over 13 will be allowed to use Google+ after the beta period is over! (Can anyone from Google confirm or deny this?) 

My only doubt is that, considering Google+ is Orkut's successor, it'll remain 18+ like Orkut, because Google+ involves making information public. But what's different than a Google profile? I can view anyone's Google Profile regardless of their age (so far that I know), and see any information they've posted!