Regarding veganism, from my experience training rugby back in England and training sprinting/handball now... it's really dependent on the individual.
For some, it works great, they get more energy, healthier, lose weight etc., for others, they have the complete opposite - vitamin deficit, lower energy, weight gain from over consumption of carb and lower power. I wouldn't advise anyone who has a physical job and then trains to follow a vegan diet as they simply won't be able to get enough protein to maintain body/muscle mass. It also seems that it works better for older ppl than 18-30s, probably because of the reduced inflammation. Ethnicity and genetics also plays a part with diet. Northern European heritages tend to be able to process dairy and meat well, grains bad (specifically gluten), whereas opposite with others, although this is a huge generalisation.
The environmental issue with veganism is that what you save on food production energy, you spend on food miles/transportation (not much soy protein is produced in europe), unless you live on local turnips

but everyone makes their own decisions and has their own boundaries and anyone not feeling great should experiment and run tests on themselves and follow the data - n=1, we're all different and have different nutritional and exercise requirements, find what works for you.
Any movement or activity at any age is good and good on everyone who does, don't need to be an athlete. The ultimate aim is to be healthy enough to enjoy life with different generations of family as long as possible