How can I become qualified/licensed to install hot water boiler systems as a job?

I live in Pennsylvania, have an associates in science mechanical engineering degree and am 60 years old. I just dry fit a two zone boiler in one of my houses and discovered it was a lot of fun. It has a Beckett Aqua Smart 2600, Tjernlund UC1, and MEC1E controls and a Taco SR502-4 relay. It’s power vented through an exterior wall and the controls are tied into the domestic water heater with a Tjernlund WHKE. I only mention my age because I see some websites refer to apprentices and I believe they have maximum age restrictions that may disqualify me. I've tried goggling my question and only seem to find training for industrial systems.