One of my referee colleagues took up refereeing at 14 as he was pants at playing football he passed the ref exam at 16 and now referees in the champions league. There wouldn't really be an age limit to assist at coaching but offer your services and do your coaching badges in the USSF. I was late to coaching taking up an offer to assist a girls team as they required a goal keeping coach within 6 years I was the goalkeeping coach to an under 16 Scottish international team.. So it can be done and you could reach a reasonable standard with commitment.
The requirements to be a coach vary widely from area to area, and from organization to organization. You would need to contact your local leagues for the correct answer.
I have been involved with leagues which require all coaches to take a USSF coaching course. I have also been involved in leagues which allowed fairly young teens to unofficially coach young players. In those leagues, though, an adult had to be the official coach, and be present at all games and practices.
I have been researching this a lot. What is the minimum age for coaching soccer? I cant find it anywhere. I live in Utah but i figure its USA wide for coaching laws