True, anyone can technically run. But in practice, a fresh-faced new candidate going up against a well-funded incumbent will very very rarely win. The few times it has happened the incumbent either didn’t take the threat seriously (AOC), or the incumbent was involved in a big scandal.
For your well-intentioned version of America to exist in reality, we’d need to do a few things: overturn Citizens United, require that all elections be fully publicly funded, ban private political donations, and stop letting elected officials draw their own maps. Until then, the gerontocracy will go on.