If the system has never had refrigerant added, it could be that it just needs topping up. It is not uncommon for a system to lose an ounce or so a year. In 13 years this means about half the refrigerant is gone.
Even if there is a leak, you will be far better off and save money by repairing the leak and recharging with R12.
It is very common for people at the slightest hint of an a/c problem to want to change it to 134 (the "new" freon.) This is in most cases false economy. To convert the system correctly, it should be thoroughly flushed, change filter drier, drain compressor and flush with Ester, add Ester and, of course, repair whatever problem there was that caused the problem in the first place. If you stay with R12, on the other hand, you simply repair the problem that you must repair any way and charge with R12. This is in almost all cases the least expensive way to go PLUS you will retain the cooling capacity of the R12.
When changing to 134 you lose about 15 to 20% of the cooling capacity.
What you should do is look for a leak. If there is a leak, fix it and recharge. If there is no leak, then top it up with R12.