Hello!

As all we know, children can do overstay without any fine. But I always see 2-3 different age ranges: some sources say, not over 7 yo, some - 12 yo, some - 14 yo. Pls, tell me, which age exactly. And is there an authority source about this rule?

There is no fine for children UNDER 15 years old.

I can't find a link to that, but a person on overstay with children was only fined for the days the child was 15 years old.

Under 15 has always been the rule mentioned here.

Have never seen an actual written quote so it seems to be an unwritten rule

Yep, but even article @ Thaivisa tells about age of seven: http://www.thaivisa.com/visa-overstay-thailand.html

Some parents don't believe me when I advice them to leave their children at home and save money and nerves

I can't find any mentioning of age 7 in the link you provided.

Look at the screenshot : https://www.evernote.com/shard/s6/sh/7a68f0dd-2c55-4538-9c2f-1502a113f7a5/c48d56ce777615867c6be394f5f4abe2

Ah, yes the title. (Who ever reads that)

That information is wrong and outdated.

From doing some googling it appears the statement of "....children under 14 years old are not fined ..." comes up often. Like in this 2008 Issue & Answer from the Phuket Gazette...answer given by a Phuket immigration officer: Link

I can't find an offciial source.

Best source would be to contact the government information line at 1111 and ask the question. They will contact immigration and provide the correct answer.

There is nothing in the Immigration Act about it but I wonder if the Criminal Act or whatever it is called has a clause indicating that children up to a certain age are not punishable for some violations of laws.

Here's a Link and partial quote from the Tourism Authority of Thailand website which says:

The penalty for overstaying your visa is typically 500B per day, with a 20,000B limit. Fines can be paid upon departure at the airport. If you've only overstayed one day, you may not have to pay any fine or you may have to pay 1,000 baht, depending on the current regulations. Children less than 14 years old who are traveling with a parent or guardian are not required to pay any fines for overstays.