The fare basis is provided , so you might be able to get it done by TA overseas, and certainly in BKK. Can't do it on the net for sure!
The link (for others interested) is here:
http://www.thaiair.com/Promotions/Sp...youth_fare.htm
Fares are usually tax exclusive. You can make a dummy domestic booking and see what the taxes are. From the THAI page...
Note : YQ surcharge and Airport Tax will be added as applicable until further notice.
Enjoy, I wish I was younger than 24 again
Cheers, TK