Trees tend to start out with a tap-root, and might favour that while conditions for a fibrous spread of fine roots aren't yet optimal.
But it varies a lot (young trees for sale in garden centres differ according to if they're grown to be sold potted, bare-rooted or bound in a hessian-type cloth), different species favour different tactics due to the ground-types they've evolved to expect and arid/exposed/restrictive conditions will create further differences.
Not sure this is a thing to worry about here. Perhaps (given the recent Mephday
cactus example and the emphasis on type-specific branch-growth rules) the generation of roots can be acutely personalised to a type, too, proportionate to the canopy. The whole underground competition between roots (existing or developing) and dwarven excavations (existing and expanded) could be a whole lot more complex - like passages being intruded into by roots (Raiders Of The Lost Ark opener, and anything that has an abandoned SE-Asian temple ruin as a location setting) and the destabilising of trees by loosening their anchoring and maybe toppling them from below (or letting them topple innthe next stiff breeze).
I expect they'll just fake [SELECT_AGE tokens]. They're going to fake naked dwarves and that has an actual gameplay impact, so not showing bearded babies is a no-brainer.
Just give all dwarves beards (
regardless of age,
regardless of gender,
regardless of clothing) and you can solve various other problems.