Um - I'm 45yo and still surf - plenty of blokes on here in their mid 50's and a couple of 60yo's...
Age is not the reason to take up SUP (666 in the surfing threads

).
Perhaps try a longer/wider/thicker surfboard that will give you a bit more paddle benefit to keep your wave count up. That's what I have done as the fitness decreases and the weight creeps up...

But at the end of the day, if SUP appeals post this question in the SUP thread and no doubt you'll get some good advice.
All the best and welcome to the 'Breeze!