My son travelled to Thailand aged 17 (just). We had lived there for three years and he was returning after 18 months to stay with school friends.

No issues on arrival in terms of immigration and customs. He didn't have a credit card and only limited cash. He did have a VISA debit card, which looks like a credit card, and he was prepared to assert that it was. he was met at the airport by friend's family on arrival.

He could speak some limited Thai ( his Mum is Thai), and very familiar with the country. Overall, a painless arrival. Many western teenagers look much older than their Thai (indeed Asian ) age peers.

My Thai niece (now an Australian citizen) visited in Jan and I arranged accommodation. Some hotels noted they would accept her staying as an under 18- year old. We booked her into. Where we stayed with my credit card to pay the bill, and we spotted out the costs between families when she returned. So hotel accom might be an issue in some places. Others may not mind.

Hope this helps, cheers, TK