Hostels

ooh this thread could turn interesting.
The law in Thailand is that you must be 18 and you must have a valid passport. <- this won't stop you being given permission to enter at immigration. It only applies to booking just about anything. I think the law for drinking is 20 but riding a motorbike is 15 and buying cigarettes has no age limit. Just like any western country the age restrictions are all over the place.
I have never seen any business ask a tourist for ID. Only the Police seem to do that and they only want to see you have a valid passport.
I'm of the opinion you won't have any problems anywhere but if you look 17 it might raise an eyebrow. And that's about all it would do!
Lastly - no one will leave you on the streets if you can pay for a room

You won't be refused anywhere.

Must be schoolies week coming up.

Tourists are often asked for ID when they check in at hotels/guesthouses even the cheap ones

Often? They always want ID. It's a requirement of any hostel or hotel. At least in any city. Out in the middle of nowhere they might forgo it.

Every place we stay at want the basics-- name,nationality, passport number. Not how old you are. And for us, anything over 500 baht is luxury.

As to getting into the country, over 16 is OK:

http://www.suvarnabhumiairport.com/en/984-suvarnabhumi-unaccompanied-minor

At accommodation, they just want your passport number for their records and to see your picture on it.

You must of course have at least 6 months validity on your passport for the date when you leave Thailand.

No, you don't need six months on your passport after you leave. Its on the IATA/web site.

westy is correct - you need six months on arrival not exit. But I think cyberia meant you must have six months because of when you will leave Thailand