Colluseum underground tickets

Tickets go on sale 30 days prior.

Some more info. on this thread:

https://community.ricksteves.com/travel-forum/italy/rome-colosseum-ticketing

Yes, it's too early.
That said, I would not get your heart set on seeing the underground. The coopculture tickets for those two, reasonably priced tours will disappear minutes after they become available...which will be in the middle of the night, depending on the release times for the various time slots. It is, hands down, the most in-demand ticket for any attraction in Rome.

So to avoid disappointment, consider one of the lesser-purchased tickets which get you inside but you'll view visible parts of the underground from above, and you'll tour independently with an audio or audiovideo guide. Some suggestions in this thread:

https://community.ricksteves.com/travel-forum/italy/colloseum-sold-out-for-may

I've been twice, both during the many, many years that the underground wasn't even accessible to tourists, and haven't felt a burning need for a 3rd stab just to do that piece.