The first thing to know is that “obscene” (that is what the authorities, consider indecent, not what you consider it to be) material in Thailand is illegal. You can be sent to prison for 3 years. Yes, even if you just use your camera phone to take a photo or a video just for your personal use. See the section of Thai law quoted below.
According to Section 287 of Thai Penal Code “Whoever (1) for the purpose of trade of trade or by trade, for public distribution or exhibition, makes, produces, possesses, brings or causes to be brought into the kingdom, sends or causes to be sent out of the kingdom, brings or causes to bring, or spreads by any means whatever, any document, drawing, print, painting, printed matter, picture, poster, symbol, photograph, cinematograph film, audio or video tape or any other thing which is obscene, (2) carries on trade, or takes part or participates in the trade concerning the aforesaid obscene material or thing, distributes or exhibits to the public, or hires out such material or thing; (3) in order to assist the circulation or trading of the aforesaid obscene material or thing, advertises or spreads the news by any means whatsoever that there is a person committing the act which is an offence according to this Section, or advertises or spreads the news that the aforesaid obscene material or thing may be obtained from any person or by any means, shall be punished with imprisonment not exceeding three years or fine not exceeding six thousand baht, or both.”
Of course many people can and do take photos or video for their own private use at their own risk. In many countries including the US, Canada, New Zealand and many EU countries immigration can search your phone and laptop. It is up to them to decide, if they think anyone depicted is under age.