Making fruit rarer and more rewarding does, in general, tend to promote larger bodies. If there's more energy in the fruit than you can store in your body, not only does the extra nutrition go to waste, but you also have less reserve to see you through to the next meal you find.
"Oldest" is perhaps a misleading choice of words to describe what's being selected for that highlight. All it's looking at is the critter's generation number -- not how long it's been alive.