Most importantly, do not go faster than you feel comfortable.
I think for those of us reentering or starting biking at an older age, speed and control downhill is particularly scary.
I restarted when I was 58, and getting my speed up going downhill was/is a real challenge.
On the Ride the Rockies, I go so much slower than many of the younger folks on downhills that I have to watch because I can be a danger to them.
I am now up to a comfortable 40 mph on certain downhills - when I can see the bottom and I know the road either levels out or goes uphill.
On real long downhills (i.e., a drop of 4,500 feet from Rabbit Ears Pass into Steamboat Springs, CO, over about 8 miles), I just stop every now and then and get myself together for another go!!
I think that once you miss those teen and age 20+ years of learning to go fast and be comfortable at high downhill speeds, it is a real chore to make up those years at a later time.
So, be safe, go your own speed, and enjoy.