I think the point is that one would expect encouraging authenticity to be appreciated, if the hobby were mainly about history, but in fact, encouraging authenticity is often discouraged.
I realized that disappointment years ago. I used to think that reenactors and museum living history people were so cool, because they had a boundless interest in history and loved to encourage that interest in others.
Then I realized: what many were really doing was encouraging you to be like them. No more, no less. As long as they were more interested than you, they encouraged you to improve. But if you became more interested than them, they switched to discouraging any further improvement or especially any attempt to point out potential improvements. As we've seen right here in this thread.
As far as age, I'd say it's just one of many factors. Events rarely if ever have upper age limits on soldiers, so one can just decide what one is comfortable with, as far as what would be typical and believable for one's age and appearance, and as far as what roles are available. Some events offer more for civilians than others. If the main attraction is a ladies tea and fashion show, a male civilian may feel like a fifth wheel, and the military is the best option. I've been to other events where civilian roles are an integral part and one can search the census for someone who's a fair fit as far as age, sex and ethnicity, and portray either him or a fictional/typical person with the same demographic.