Underage chicas is a bad topic I like to avoid. If you get caught you can probably count on the cops at least emptying your ATM and credits cards if your carrying them, and then you would be considered getting off lucky.
I highly doubt you'll be killed in prison for mistakenly or on purpose having a session with a slightly underage chica 16, or 17, you might even get sympathy from other inmates saying 'what a tough break!' Now if the underage girl was 5 years old,then you might be KILLED? [ I don't know for sure because I never been in a TJ prison, and plan on keeping it that way]
It's not worth the risk in Tijuana, besides there are a lot of nice young 18, 19 year old girl on the north side of the alley who wear little school dresses with small tits and high heels that will help you fulfill your high school sex fantasy. In fact I just did one on my last visit to TJ about two weeks ago, damn I already forget her name, but Flor comes to mind who also works there, and Vicky a innocent looking girl who always wears a dress accross the street.
The dangerous areas on on lower Constitution where some of these girls just look too young, but say they are 18. There are certain hotels a little outside the zona you should avoid too. I forgot the name, but probably wouldn't post it even if I remembered it.