It is normal for air conditioning systems to leak refrigerant over time. Some systems may lose several ounces a year, which means after several years it is not unusual for your air conditioner to need some additional refrigerant.
If your AC is blowing warm air or just not cooling your home as well as it used to, it may need to be recharged with additional refrigerant. Many homeowners may not be able to tell their air conditioner is in need of refrigerant, but the low charge could be affecting its efficiency. Also, poor cooling could be a result of something else.
Therefore, it is recommended that a skilled professional with the proper equipment tests your system and diagnoses your air conditioner. If you have a maintenance agreement, this is part of your annual air conditioning tune-up.