Adding a little bit more to it, and I don't know CA liquor laws, the actual possession of alcohol a person under 21 may or may not be a crime. The reason I say this is that states can be aligned in one of the following rules for alcohol

1) Illegal to sell/serve to a person under 21, illegal to purchase if under 21, illegal to possess if under 21 and illegal to consume if under 21

2) Illegal to sell/serve to a person under 21, illegal to purchase if under 21, and illegal to consume if under 21

3) Illegal to sell to a person under 21 and illegal to purchase if under 21

4) Illegal to purchase if under 21

5) Illegal to sell to a person under 21

Any one of the five variations above met the requirement that the Feds set down when they tied the highway funds to a "21 year old drinking age". If you notice the last 4 in the list of options, a person under the age of 21 can posses alcohol, and only in the first two is it illegal for the person to consume the alcohol. Many states have specific exceptions to the consumption and serve rule that does make it legal for a person under 21 to consume alcohol. For example, many states allow a person to drink if a person over the age of 21 orders the beverage for them and pays for it. Typically, this rule is limited to if a parent/gaurdian buys a drink for a child or a husband/wife buys a drink for their underage spouse. It generally does not apply to buying and providing to persons unrealted to the purchaser.

So to say that possession is illegal is not necessarily true.

Now in the OP if the items were in his bag and not in the baggie, then TSA had the right to detain him for the search, but turning him over to LEO's may have been too much, but that also depends on the law of CA. By that I mean if CA has a law that requires persons to report any crime that they see occuring then the agents would be justified in turning the case over to the LEO's. However, I do not know the actual laws of california and thus cannot provide a correct assessment on this situation.