A few years ago I ran across a nifty little chart that attempts to compare dog age to human age.

Relation of dog to human years

Dog Human
  6 months
  8 months
10 months
12 months
18 months
  2 years
  4 years
  6 years
  8 years
10 years
11 years
12 years
13 years
14 years
15 years
16 years
18 years
20 years
21 years
  10 years
  13 years
  14 years
  15 years
  20 years
  24 years
  32 years
  40 years
  48 years
  56 years
  60 years
  64 years
  68 years
  72 years
  76 years
  80 years
  88 years
  96 years
100 years