All horses undergo an "Age Change" when they turn 3 and again when they turn 4. This happens instantly on the night they age. The age change results in an increase in ability, i.e. the horse will have improved TT's. As every other horse also undergoes this age change, it is the magnitude of this increase that determines whether the horse increases its competitiveness as an older horse. Horses do not recieve an age change between 4 and 5, 5 and 6 or bewteen 6 and 7. A horse cannot race past 7 and is automatically retired at the end of it's 7th season.
In raceclubs terms, 4 months equates to a virtual horse year.
1st August to November 30th
1st December to 31st March
1st April to 31st July
The age change occurs on the first day of each virtual horse year (4
Months)
So in a calender year a horse who begins life as a 2YO will become a
3YO and 4 months later a 4YO.
Unfortunately it is diffcult to state the percentage increase required. You will need to keep up to date with the prevailing standards to determine if the increase your horse(s) received was enough to keep them competitive. One season it may be 12%, another 15% or indeed even higher. Obviously if the horse was above average before the age change, then a lesser age change may suffice to keep the horse competitive.
Grade 1's recieve age changes in the region
.................... 10% to 15%
These %'s are a guide only, there can be age changes outside this
range but most horses will fall into this 10% to 15% range.
With the possibility of poor, average or good age changes for 2 and 3 year old horses near the end of the season, you should factor in this uncertainty when purchasing horses. You are taking a chance that the horse will still be competitive after the impending age change.