For Elder Dragon, is enemy base consider a location I can target?

Yes, an enemy base is considered a location, and you can target an enemy unit located there with Elder Dragon's ability.

According to Rule 107.1.b, a base is a location. Elder Dragon's ability allows you to "choose up to one enemy unit at each location" and deal 1 damage to them. Since a base is a valid location, any enemy unit occupying that base is a valid target for the ability.