Age (henceforth referred to as Oldness) can be measured in years, but it can be hard to tie concepts like "old" and "young" to a certain number of years. I like to measure age in the ratio between the number of presents you receive on your birthday and the number of years you've lived.
Most people will start out with a ludicrous 15-20:1 ratio, which then drops sharply until it reaches a 1:1 ratio (usually somewhere between 10 and 20). After 20 there is a sharp drop in the number of presents received, leading to a high increase in Oldness. This seems to coincide with the age when the government has decided it is OK for you to enjoy a glass of wine at home, which is generally seen as a sign of Oldness.
At this point the presents will remain static, while the years lived will steadily increase. On your 30th birthday there will be a sharp increase in the number of presents received, accentuated by the sharp drop back to normal levels one year later.
By my calculations you should reach a presents to years ratio of 1:5 between 25 and 30. And that's when you're "old". You won't realize this until you're 31 though :P
Ps. My current ratio is 1:4 - still young, in other words :D Ds.
