If an expert pilot or covert operative or other specialized profession can be referred to as an "ace", is there a word that describes someone who is less experienced or lacks some of the expertise? Is it sequential? What pattern does it follow?
I don't believe there is a pattern to this, but there are synonyms, antonyms and ways to phrase the opposite to varying degrees of status. For instance, if we take your example in a number of forms:
Then we can construct some to refer to those lacking some skill but aspiring:
And further, construct some to refer to those that, despite any involvement and experience, don't have skills at all really:
I don't believe there is.
The term most likely comes from the Ace being the highest valued card in a deck of playing cards.
The playing card most likely got its name from the French word 'as' meaning 'a unit', since the ace is a representation of a single pip.
However, the lowest valued card is the 2 of clubs. Maybe you could refer to a lesser experienced person as a 'deuce'.