MSDN subscription for a 15 year old, i don't even know if they allow people of that age
. First off all you should consider if you really need a MSDN subscription, if all you really want is Longhorn builds, then just leave it and search the net on how to download Longhorn from somewhere else.
If you are a software developer MSDN subscriptions can be usefull, however they aren't cheap. Because you want Longhorn builds you would probally want a MSN Operating Systems Subscription (
http://msdn.microsoft.com/subscriptions/buy/choose/ ), i don't know if you already have a legal copy of any Microsoft Visual development tool because then it could be a bit cheaper, else it will cost you at least $699 (
http://msdn.microsoft.com/subscriptions/buy/prices/default.aspx#os ).
In other words, unless you are a professional developer and have the money you are better of getting Longhorn from other sources (but can't talk about that here, warez is forbidden at this forum).