TL;DR at age 75 you deal +40% damage compared to age 20.

It's impossible to start The Slums at anything other than 20 years old in a legit way afaik, so this is only useful for the other 4 levels.

Here are some damage number examples to further show this. This was all tested on the bouncer of The Club (the very first enemy) who has 210 health and 315 structure. The raw values were obtained with Cheat Engine:

AGE 20 HEALTH DAMAGE:
10 = 1st Light Attack
20 = 1st Heavy Attack
20 = Sweep
20 = Slide Kick
30 = wall splat close
 0 = wall splat far

AGE 75 HEALTH DAMAGE:
14 = 1st Light Attack
28 = 1st Heavy Attack
28 = Sweep
28 = Slide Kick
42 = wall splat close
 0 = wall splat far

AGE 20 STRUCTURE DAMAGE:
12 = 1st Light Attack
24 = 1st Light Attack (Counter Hit)
24 = 1st Heavy Attack
48 = 1st Heavy Attack (Counter Hit)
36 = Sweep
72 = Sweep (Counter Hit)
36 = Slide Kick
72 = Slide Kick (Counter Hit)
24 = wall splat close
42 = wall splat far

AGE 75 STRUCTURE DAMAGE:
 16.8 = 1st Light Attack
 33.6 = 1st Light Attack (Counter Hit)
 33.6 = 1st Heavy Attack
 67.2 = 1st Heavy Attack (Counter Hit)
 50.4 = Sweep
100.8 = Sweep (Counter Hit)
 50.4 = Slide Kick
100.8 = Slide Kick (Counter Hit)
 33.6 = wall splat close
 58.8 = wall splat far

I haven't dived too deeply into damage multipliers, but the Counter Hit double damage gets triggered when you hit the opponent during the startup of their move, interrupting it. It only seems to apply to the structure and not the health.

Btw I managed to play at age 85 with a bug, so I checked if the damages got even higher and... they did not.

I've made an age 75 save file with the full skill tree ready for Individual Level speedruns. Can't upload it to the Resources page because I lack permissions, so here it is: https://www.mediafire.com/file/s36j2wrijq1li70/Sifu_savefile_age20_age75.zip/file