OP - Nothing wrong at all with taking up martial arts at any age….BUT…we all need to recognise our limitations. For my 60th birthday, I did 3 rounds of moderate sparring at a top Muay Thai gym in Bangkok…I trained for about 3 months to get fit enough, and I was able to draw on many years of Thai boxing experience (albeit I hung up my gloves many moons ago). Not sure I could do it when I reach 70 though ????
If you are new to the sport, forget all about sparring, just pop into a Muay Thai gym and sign up for a non contact fitness class, or take private lessons. As others have said the boxing bit is easier than the kicking, but Muay Thai is a bit easier for us oldies given the emphasis is on low kicking. The coaches should be teaching you the basics - how to punch/strike and how to avoid getting hit. Practicing on the bags and with a trainer using focus mitts is a great calorie burner, brilliant for fitness and handy for self defence.
All this assumes that you are in good health - maybe medical check before starting training. Good luck with your training.