You are never to old to have fun, be it Pokemon, Call of duty, or watching professional sports. When trying to make a competitive team i think of these following things:
-What is the goal/core strategy of my team? (do i want to stall, or do i want to win with hard hitters, are some examples).
-What pokemon are best suited to accomplishing the goal of the team?
-Are they effective at what they do? (sweeping, stalling, hazards, set up, ect)
-Is there some synergy with my team/ do my Pokemon work well together?
Also, know your outs in a match. For example if their Scizor just got of a Swords Dance, think of a new strategy that involves stopping it.
Competitive battling is something that comes is time. There is a bit of a learning curve in terms of EV spreads, prediction, IVs and meta calls. I've found that in competitive battling you can have flawless Pokemon with perfect natures, EVs movesets and whatnot, but if you cannot predict, to a degree, what your opponent will do then winning will be more difficult.