How to loop in NetLogo?

Whenever possible, I'd suggest paring down your code to a MRE to a) make sure that users here can run your code (without your files, for example, this is not really viable) and b) to see if reframing your question / goals in simpler terms would help get things working- at least that's what works for me!

I think that you might find foreach useful here as a way to loop through your desired simulations instead of manually tracking the number of iterations. For this example, assuming num-user is a numeric input widget on the interface, the setup below will determine which of the ages to process:

globals [ ages-to-run ]
  
to base-setup
  ca
  ; Determine how many simulations to run, based on user input into
  ; 'num-user' numerical input widget on the interface
  ifelse num-user < 1 or num-user > 3 [
    print "Incorrect number of simulations indicated"
  ] [
    let possible-sims [ "2" "4" "8" ]
    set ages-to-run sublist possible-sims 0 num-user
  ]  
  reset-ticks
end

After running the above, the ages-to-run variable will contain ["2"], ["2" "4"], or ["2" "4" "8"]. Next, you can iterate over those desired ages to run your simulations (a little more detail in comments):

to run-simulations 
  if ages-to-run = 0 [
    print "Simulation setup not complete"
  ]
  
  foreach ages-to-run [
    ; This is the loop proper where each "age" is iterated.
    ; All of your simulation calls (manual variable resetting,
    ; etc) should all go within this loop.
    current-age ->
    print word "Running simulations for age: " current-age
    let file-to-load ( word "./L_" current-age ".asc" )
    print file-to-load
    clear-turtles
    ask n-of 5 patches [ 
      sprout 1 [ 
        pd 
        set color runresult current-age + 55
      ] 
    ]
    repeat 20 [
      ask turtles [
        rt random 60 - 30
        fd 1
      ]
      tick
    ]
    print ( word "Simulations complete for age: " current-age "\n" )  
  ]
end

Running that code above with 3 entered into num-user will run a simulation for each age (different colors indicate different runs):

So to run simulations proper, all your per-age code should go within that foreach loop indicated above- and be careful, as you were in your question, to not reset global variables.