However you don't roll for very long as your back hurts...according to you. When you say physical contact it's not actually that long is it, as for being with people younger well, that doesn't mean they are any fitter than you. Many people who train are older than you, also have full time jobs and families.
Oh and if you are wearing Muay Thai shorts for MMA please don't …. unless you are wearing shorts underneath, it's never a good look when your undercarriage drops down.
You seem unreasonably biased against doing BJJ in a Gi, I think you have either been ill informed or have badly misunderstood BJJ.