Training age is the number of years you have been doing regular, serious strength training. It is not the time since your first gym membership, and it has nothing to do with your actual age.
It is a rough but useful measure, because it describes how far you are from your personal limit. Beginners gain much faster in their first year than intermediates, and with every year the possible gain gets smaller.
What it means for bulking
Iraki et al. 2019 distinguish two groups: beginners and intermediates with about 0.25 to 0.5 percent gain per week and a surplus of 10 to 20 percent above maintenance, and advanced lifters with about 0.25 percent per week and a surplus closer to 5 to 10 percent. A finer split by years is an interpretation within that range:
Example: Year 1 about 0.4 to 0.5 percent per week, years 2 to 3 about 0.3 to 0.4 percent, from year 4 about 0.25 percent. At 80 kg, that is the difference between roughly 0.4 and 0.2 kg per week. These values are a guide, not a study requirement.
The closer you are to your limit, the more of every extra calorie ends up as fat. A beginner benefits more from the upper end, an athlete with many years of training more from the lower end.
Where it matters less
Not everything depends on training age. When estimating reps in reserve, training experience made little difference in an analysis by Halperin et al., and experienced lifters misjudge too. And after a long break, people returning to training often rebuild muscle they once had fairly quickly in practice, even if their training age looks high on paper.