I don't know how to read the script that well but this looks a bit like there's a correlation between aging and which character body piece model is being used.

body_part = torso
textures = {
diffuse = "gfx/models/portraits/decals/male_body/decal_male_body_fat_01_diffuse.dds"
normal = "gfx/models/portraits/decals/male_body/decal_male_body_fat_01_normal.dds"
}
alpha_curve = { #controls decal alpha relative to gene strength. Will give a linear interpolation if left unspecified
#character age%, decal alpha
{ 0.0 0.0 }
{ 0.5 0.0 }
{ 1.0 1.0 }
}
blend_modes = { #overlay/replace/hard_light/multiply
diffuse = multiply
normal = replace
# properties =
}
priority = 0
age = age_preset_child_fat_decal
}
# Gaunt body from 0.5 to 0.0 gene value
setting = { attribute = "bs_gaunt_1" value = { min = 1.0 max = -1.5 } }
setting = { attribute = "bs_body_gaunt_1" value = { min = 1.0 max = -1.0 } }
decal = {

Copied from the 01game morphs file found in this path C:\Program Files (x86)\Steam\steamapps\common\Crusader Kings III\game\common\genes

I like to play with cheats so my guy was already a bit modded up to max stuff out. He was age 30 and by the time I noticed his body was changing, he was already fat, immortal and age 70. I think there is something to be said about the script above and what the dev said about "effective age" vs "real age".

Additionally, age related events still trigger such as "Graceful aging" which is something you get from completing one of the lifestyle trees and only starts occurring once you reach an old age.