I think we need to stop calling it "making balance" because that's not exactly the problem. Unbalanace riders are natural, not every climber can cobbles and not every sprinter can climb etc etc. Specializations exist for a reason. Otherwise we'll end up with a ton of very similar riders with only minor differences in skills. Personally i like the different variations in riders.

There just needs to be some scaling in upgrade cost. It's just too cheap to raise the highest stats compared to the lowest ones. It's one thing to restrict by age but a more efficient system would be to restrict by attribute value.
Just off the top of my head:
<60 60% cost
<65 75% cost
<70 95% cost
<75 105% cost
<80 120% cost
<85 200% cost
You know something along those lines. So instead of it costing more because a rider is older it costs more because it is worth more! You can then add rider age restrictions afterwards which is beneficial to younger riders rather than punitive to older riders. For example:
+30 50%
27-39 75%
24-26 100%
22-23 110%
20-21 125%
18-19 150%
Now the system rewards youth and punish age whilst still allowing growth during the "peak" years of a career. Combine that with the high cost of upgrading higher stats and you should get a more balance progression of riders. Younger riders will get much better in the key stats and then as riders get older they get better support stats. This is a much more realisitc development trajectory.

And i just threw these numbers down whilst writing so they would need some testing and adjusting but at a glance it seems not too far off working.
I chatted a bit via PM with Oldfart about development and the "free upgrade". I said at the start we needed to run a season and see how it worked. Season done and imo the system needs some serious work. The above is one suggestion to kind of re-work it from scratch. Whether the system is built from scratch again or the current one adjusted the relative costs of raising high and low stats MUST be altered and i think youth should be rewarded rather than age punished because as shown it's almost impossible to raise a stat above 80 once a rider turns 22/23.