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

Unbreakable low throw for the CPU.

This strat only works in the EU version of the game, and Glen (ally available starting from mission 34) can do it. Opponents using the blue Concentration SPA buff will still break it. Players can also break it.

Slide Kick cancels over and over are slightly faster than simply running, here's a comparison:

EDIT: 3-way comparison with a sword due to request

• starting the timer from the first frame of the cutscene (NOT the loading screen) OR in case of no cutscene the moment you take control your character

AND:

• stopping the timer on the final loading screen

...is the most reliable way to count IL timers accurately across PC/PS2/Xbox afaik. This is compatible with both the upgrade screen and the retry screen

As shown here in the IL run I just submitted:

These are the 2 major strats:

• don't upgrade to Aerial Strike Level 2. The regular one is much better for both movement speed (Aerial Strike repeatedly) and combat (easier Tornado Throws which is strong and fast)
• wall splat properties are glitched in this game and Focus Aerial Throw Level 3 can exploit this by inflicting insane damage. The purple witch in particular has by far the highest health (4500), defense, and health recovery in the game (excluding immortal agents) and yet she dies in one throw, skipping one of the two fights completely

Also there are some minor optimizations here and there:

• save the fake red witch in the middle for a fast way to trigger the next cutscene, plus we get to damage our ally
• I use the Jackpot item upgrade to kill the red witch quicker
• killing the red witch saves time on unnecessary cutscenes, loading screens, and dialogues
• the first throw on the purple witch is for wall positioning reasons, the second one is for the kill
• you don't need to keep the focus ON for the entirety of a move that requires it; you can often just start a focus move and then stop focusing immediately, so the game has less slow motion
• reminder that the zoom reset button to quickly turn the camera around exists (would recommend to use during Aerial Strike)