I am 63. One bike I ride is an FS trail bike with riser bars for aggressive trails riding and some DH. I also have a dropper post so, when my seat is at full height it is level with the top of my stem.
I also have a hard tail XC bike with a flat bar. My seat is level with the top of the handlebar. This is mostly for speed and efficiency.
The seat height measurement I believe is more important is that I get a full leg extension when my foot is at the bottom to cycle. The handlebar height is set to balance my weight evenly between the tires. This way I can slide around on the seat to shift my weight front to back as necessary.