John,
I'm not so young myself.
I'd expect some leakage at the compressor shaft and suggest you shine the black light on the A/C plumbing as far as you can, if only to rule out other leaks before attacking the compressor.
Another possibility is a closed expansion valve; the only way to know that is from the system's pressures. The charge and compressor may be fine but reduced cooling results if the expansion valve is corroded.
My old 300SD had lame cooling this spring so I had it "topped up" (after
9 years) and it now cools perfectly. I considered having it converted to R134a but the subject never arose as R-12 was added. The technician told me that the price of R143a had more than doubled in the last year or so and that its cost advantage (over R-12) had nearly evaporated. I write this so you don't get "sold" a conversion to R134a on the basis of future cost saving.
Freon IS dangerous and it's now illegal for non-licensed mechanics, including DIY, to open the system. So you can research the parts cost but not fix it. Suggest you return to the shop that added the dye, play dumb, and see what they tell you. After you get their estimate remember that the summer heat will only last another 45 days or so!
Tom