How does "gaining the right" to advance classes work?

After playing more and eventually looking it up, I discovered that different classes simply unlock the next tier at different levels. From this guide:

Here is the breakdown for when each character class can be promoted at their earliest:

• Mercenary promotes to Myrmidon (Level 7), who then promotes to Dread Fighter (Level 10)
• Soldier promotes to Knight (Level 7), who then promotes to Baron (Level 10)
• Archer promotes to Sniper (Level 7), who then promotes to Bow Knight (Level 10)
• Cavalier promotes to Paladin (Level 7), who when promotes to Gold Knight (Level 10)
• Pegasus Knight promotes to Falcon Knight (Level 12)
• Cleric promotes to Saint (Level 12)
• Male Mage promotes to Sage (Level 12)
• Female Mage promotes to Priestess (Level 14)