How many years a client has trained consistently and appropriately, which predicts their rate of progress far better than their calendar age does.
A 45-year-old who has lifted seriously for six months is a beginner, and should be programmed like one. A 22-year-old with four hard years behind them is an intermediate, and linear progression will not work for them.
The rough tiers: under a year is beginner, where load can rise session to session; one to three years is intermediate, where progress comes weekly and needs a structure such as double progression; beyond three years is advanced, where progress is measured per block and periodization is not optional.
Count only consistent training. Two years of turning up twice a month is not two years, and the client will progress like the beginner they are, which is good news to deliver.