Right now a 18yo Creation will hit around 90% of peak by 23-25 dependent upon learning speed, and mostly that will be at 6.0 or higher for creations that experienced managers keep. 25 year old creations are unlikley to hit peak before they decline.
The ideal would be for 25 year olds & 18 year olds to be "open ID ready" at around the same point give or take a little for learning speed disparity.
This gives 25 yr old builds some value beyond just being throwaway creations new players use until they get used to the game and figure out that 18 Year olds cooking for 3 months + are the way to find success.
18 year olds having less downsides for fighting, namely the raised points of lost skill gain while recovering from fights & the damage to their chin before they have the skills to properly absorb & evade are what would need to be looked at to encourage them to fight rather than cook.