The birth date for each player in the MLBPA associations has been set so that in Career Associations, each player's Age is increased on their actual birthday. Between seasons, each player will experience the effects of aging according to his age when Start New Season is selected. Younger players will generally see an increase in their Actual Ratings while older players will see a decrease.
Each rating has a "normal" growth curve based on the player's Potential Rating and Age in that skill. A player's growth can be accelerated, and his decline slowed down, by applying extra training time to that skill in Spring Training. Similarly, his growth will be severely impaired, and his decline accelerated by providing less than the minimum required amount of training time to a skill. However, the vast majority of a player's growth in each skill is due to aging and his potential.
In general, a player's skills will peak at about 28 years of age. However, each skill has a different growth curve and may actually peak earlier or later than 28. Although each player is unique and can vary from the norm, the following guidelines can be used when gauging the effects of aging on player skills:
1. Contact hitting (CH) is a younger player's skill. Although teen-aged players will not likely start with a large portion of their potential, they will improve quickly and should peak before they reach 28 years of age. Players should maintain their contact hitting well into their 30's before they start to decline in skill.
2. Power hitting (PH) is an older player's skill. Teen-aged players will not likely start with a majority of their potential and will not improve as quickly as with contact hitting, but will start to see greater improvements as they get stronger in their mid-20's and will continue past age 28. Power hitting is maintained by players into their late 30's, but should show a sharp decline as they age further.
3. Speed (SP) is a young player's skill and is pretty much established as a teenager. Player's may see some minor improvement in their speed as they get older, but a player's speed should peak well before he is 28 years old. Players start to lose speed as they get into their mid-30's and their speed decline is rapid.
4. Fielding ability (FA) is a skill that established early, but has room for moderate growth. Players will generally peak at about 28 years of age with their fielding ability and will maintain it throughout their career.
5. Arm strength (AS) is similar to speed in that teen-aged pitchers and position players already have almost all the arm strength they will achieve, but with room for minor growth. Arm strength is generally maintained by the player or pitcher until somewhere around 40 years of age and then will generally decline rapidly.
6. Control (CO) is an older pitcher's skill and, like power hitting, will show a slower early development with an accelerated growth as the player hits his mid-20's and continuing past 28. Pitchers will generally retain most of their control into their 40's.
7. Endurance (EN) is an established skill with nominal growth and will be maintained well into the late 30's, but will generally start to show a sharp decline thereafter.
8. Hold runner (HdRn) is a skill in which a pitcher can show moderate growth, peaking at about 28 and maintaining throughout his career.
9. Pitch ratings (FB, SI, SL, CB, CU, SC, KN) are usually established, but can show moderate growth. In rare cases a pitcher may have a hidden ability in the knuckle ball or screwball pitches, but otherwise, if a pitcher has no ability to throw a pitch, he won't learn it.