People age out but those that age out don't have the attitude of "old people", which in this case is "why would people watch this" and "this is a waste of time". (not trying to generalize older people but i think you guys know what type of people I'm talking about) Slowly esports and will become more and more mainstream (probably not completely mainstream but I can't predict the future) and more and more people will watch it.

Also, for the people that age out it seems like 3 more people join. Search viewership growth for games like League of Legends, Dota2, and CSGO and you will see that watching videogames is serious business.