I try to equate us humans as an example to this theory...
Most male teenagers go thru puberty, on average, between 13-14.
Some boys are younger and some are a little older.
In fact I've seen a kid from my high school graduation that was 5' 7" tall, grow 6" within a year of our graduation,
he is now 6' 1" - fully grown, who would have known.
So your question about the 2 1/2 old scenario is a good "average" to see the nice rack growing potential.
Remembering that some 1 1/2 year olds can have nice baskets or may only have small, stuby, spindley spikes.
Some 2 1/2 olds can only have a small basket or can have 8 points and 17-18" wide rack with 8" tines.

Second part is genetics... My father was 5' 5" and mommy is 4' 10", so I guess how tall I am???!!!
Should I be culled??!!!!