This feels really obvious to me. Of course people just starting their adult lives wouldn't have had a chance to build up much savings / acquired an inheritance. Naturally they'll buy a house and start building equity in it before investing a huge amount into the stock market. As they get older, retirement accounts and home equity will grow, leading to older people having more wealth.

An intereting analysis would be to look at what this looked like for different generations adjusting for age (what did savings for baby boomers look like when they were in their 30s?).