I am assuming that "September 28, 2012" should read 2011 as mentioned later on in your OP. It is likely that you used a Google account which you created in September 2011 and since the joining date is dictated by the managing account, the Google account, it is that date which is considered to be relevant.
If you create a new Google account, with a new GMail/Google+ profile in order to create a YouTube channel you will get the date of creation being current.
As for "wrong" ages- that is down to whatever those users input in the Google services which are since associated with one another. Google will take the lowest (ie youngest) age and apply that across all associated services in order to conform them. Probably because it is common for users of YouTube to make themselves older than they are due to the age restrictions for using YouTube. ;)
Never really having seen an issue with completely impossible joining dates I cannot comment (unless you are actually referring to the 1889 bug which is relating to age, not joining date) but the gender thing is, again, down to data somewhere across the various associated Google services.
You'd be surprised what we get up to in order to maintain a semblance on anonymity on the internet and then forget about. And Google changed the goalposts on us without notice so that we are now stuck with data which is not "true" because we were being careful but also not necessarily known to us any longer because it wasn't important TO know it. Meh.