Security of AOL user names?

[Yorkshire pud]

I'm a bit troubled about what I was told by a geeky friend of mine the other day. For those who don't know, AOL (of which I'm on the UK version-separate company to the USA version) lets you set up a 'master' screen name, and from that you can have six more screen/ user names on the same account. They naturally have separate passwords and fave places etc.. He told me that knowing any of the names he could find the master screen name, and more troubling, that he could find out all my personal details. Name, address, age etc.. He wouldn't tell or show me how it was done, so I don't know if it's BS. Firstly does anyone know this is true, and secondly is it really an easy thing to do? And can it be prevented?  Reply in PM if you wish..Thanks in advance..