According to the SRD, they get it at their 5th age category (young adult), and running an average, it increases by an average of 1.6/age category, except for Wyrm -> Great Wyrm where it always increases by 2. But that's only based on SRD dragons, dunno if others break the trend.
Age Category | Avg. SR increase | Min. SR Increase | Max. SR Increase |
YA -> Adult | 1.9 | 1 | 2 |
Adult -> Ma. Adult | 1.8 | 1 | 3 |
Ma. Adult -> Old | 1.7 | 1 | 2 |
Old -> V. Old | 1.2 | 1 | 2 |
V. Old -> Ancient | 1.9 | 1 | 2 |
Ancient -> Wyrm | 1.2 | 1 | 2 |
Wyrm -> G. Wyrm | 2 | 2 | 2 |
Precise average of SR increase per age category, excepting Wyrm -> G. Wyrm: 1.61666 (repeating 6).
Average SR @ YA: 18.8
Minimum SR @ YA: 16
Maximum SR @YA: 21