How did the people in Jesus' generation refer to the years of a calendar? What year would Jesus have said it was when he was 20 years old?
I'll point to this answer of mine from last week, though with a caveat that pretty much everywhere had its own calendar at the time.
To give a specific answer to your specific question, he would have said something like: the 10th year of the emperor Tiberius. (That's relying on the chronological info that Luke 3 gives, and ignoring the inconsistent chronological info elsewhere in the gospels, including in Luke.)