system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between a fit person's veins that "pop" so they are visible from the naked eye and eg. a grandma with clear veins on her hands? Are they a good sign of health?

For those who dont understand what I mean, typically when you see a very fit person's arms or legs, you can see veins running down them. But then if you look at an eg. Grandma's hand, there are several veins there also. She doesn't do weights, or not exactly the same amount of exercise the 'fit' person does, so whats the difference? Also is it a good sign of health?
gpt
It has to do with the mechanism by which the veins are visible. In younger people, the veins are visible because the muscles are pushing them up against the skin and there's no fat blocking them in. In older people, it's usually because the skin is thinner and becoming more pliable meaning it forms around the veins. The visibility of veins is not in and of itself healthy, plenty of malnourished people have visible veins and they are arguably not any healthier than a person of average build without veins showing. It usually just indicates a high muscle-to-fat ratio or looser skin.