Someone is holding a poker game, would it be legal for a minor to participate? This is in Texas.
Generally, it would be a juvenile offense for a minor to gamble and a crime for an adult to do so (assuming that the minor is old enough to be prosecuted for juvenile delinquency, you can't prosecute a two year old for playing poker).
There are affirmative defenses to this crime (and the parallel juvenile offense) if:
the actor engaged in gambling in a private place;
no person received any economic benefit other than personal winnings; and
except for the advantage of skill or luck, the risks of losing and the chances of winning were the same for all participants