Sir beetols, according to 4J series documentation, replacement is dependent on play and backlash not necessarily on the age and mileage. Eto and sabi:

1.) Timing Gear Gacklash mm (in)

Standard Limit
.10-.17 (0.0039-.0067) .30 (.012)

a. Set the dial indicator to the timing gear to be measured. Hold both the gear to be checked and the adjusting gear stationary.
b. Move the gear to be checked as far as possible to both to the right and to the left.
c. Take dial indicator reading. If value exceeds limit, timing gear must be replaced.

2. ) Idler Gear "A" End Play mm (in)

Standard Limit
.07 (.0028) .2 (.0079)

Insert feeler Gauge between idler gear and the thrust collar to measure gap and determine the idler gear end play. If the value exceeds limit, the thrust collar must be replaced.

3.) Camshaft Timing Gear End Play mm (in)

Standard Limit
.050-.114 (.002-.0044) .20 (.008)

Use dial indicator to measure cam shaft end play. This must be done before removing the camshaft gear. If the camshaft end play exceeds the limit, the thrust plate must be replaced.

i hope this helps.