I’m 68 and “now” 5’8” with 29” inseam. I rode 170mm for decades on my road bike. When I started mountain biking I rode 175mm.
About 5 years ago, I found an NOS Ultegra 6503 crankset with BB for less than $150. It was 172.5mm. Now I ride 172.5mm on my road bike and haven’t noticed any difference.
John