Multiple themes in the same wp installation

Hello.
I have a website that has 3 kind of visitors of different ages. 18, 21 and 23 years.
What I am trying to do is split my website in 3, so when you enter you select the section you want (18, 21 or 23) and you are redirected to a different theme, in the same installation.

In other words, I’d like to give users the choice of the theme they want when they enter to my website, something like this:

1. you enter to http://www.mywebsite.com
2. you select a theme, and you’re redirected to http://www.mywebsite.com/theme-name-you-selected/

And show the exact content (same wp installation) for every theme.

How could I do this?

Thank you.

ps.: sorry for my English