Most will be in the range of 20-40. Some will be older, but none will be kids. So take it as you will. The true number doesn't have any intuitive meaning anyway, since the likelihood of being infected is correlated with the likelihood of being symptomatic.

E.g. if hypothetically people under 20 never exhibit symptoms but also never get the disease, they won't bring the asymptomatic rate down because they won't be infected in the first place.

Averages suck in this context.