Can lower-level players use a mercenary?

At low level it cost little or nothing to hire and maintain a mercenary. For the first 50 or 60 levels the melee mercenary is quite powerful and can help you level quickly. At higher levels most magicians will probably want a healer mercenary, at least while soloing or grouped without a healer.

The mercenary levels as you do so they automatically become more powerful as you level (they don't change rank or tier though).