I work in NYC in commercial buildings. My advice to you is get every license you can get your hands on and study everything. I got into the field at 24 and got my NYC reefer license at 26. I ran into a few interviews where the age thing came up. My situation is different from a field tech where I work for the building. Not every building manger is about to start trusting a 26 year old with only a few years experience with their building. But if you know your stuff you will do fine.
I've worked along side with guys from Carrier, York, Honeywell, Johnson etc... The carrier people always seem to speak highly of the company. They are really big into safety. If you can get with them, they are a great start. Only complaint I ever heard is from the older guys about how they put GPS in the vans. Haha
Good luck with everything!
One more quick thing about this trade, you get to learn what works and what doesn't work on other people's equipment. So when it comes time to work on your own stuff you are ahead of the game.