Here is the full list of all 24 Oldage achievements.
Kill someone with a Priest
Do three damage at once
Buy one of each unit
Listen to the knight's sad music tale
Win a match with a Royalty unit
Win a match without getting an unit hurt
Get to the last tavern tier
Play an inverted match
Play a rapidfire match
Play a friendly match
Play your first ranked match
Win 10 ranked matches
Win 50 ranked matches
Win 100 ranked matches
Win 10 casual matches
Win 50 casual matches
Win 100 casual matches
Add a friend into the game
Add 5 friends into the game
Have your opponent's royalty fall off the map
Your royalty fell off the map
Both royalties fell off the map at the same time
Your royalty is the last one standing in the last cube
Spectate another match