Came across this thread last night, and had a flash of inspiration.
While it may not be a 'rule', Filipino ships will be aged according to the following formula:
Age 1-15: 0.5 pt a year
Age 16-30, refit at or before 15: 1 pt a year
Age 16-30, not refit: 1.5 pts a year
Age 30+, not rebuilt: 2 pts a year
Age 30+, not refit or rebuilt: 3 pts a year
If not refit before age 15, refit will only 'restore' 15 yrs.
(I.E., ship 4 years over (19 y.o.) = refit to 98%)
Ship refit at 10 and 20:
Built in 1910
1920, 95% (0.5 pts a year, refit to 100%)
1930, 92.5% (0.5 pts a year to 1915, 1 pt/yr after, refit to 100%)
1940, 90% (1 pt/yr, rebuild needed)
Ship refit at 15:
Built in 1910
1925, 92.5% (0.5 pts/yr, refit to 100%)
1940, 85% (1 pt/yr, rebuild needed)
Ship refit at 20:
Built in 1910
1930, 85% (0.5 pts/yr to 1915, 1.5pts/yr after, refit to 97.5%)
1940, 87.5% (1 pt/yr, rebuild needed)
Ship not refit before age 30:
Built in 1910
1940, 70% (0.5 pts/yr to 1915, 1.5pts/yr after, rebuild needed)
Rebuilds and reconstructions always 'reset the clock'