Mold and mildew grows in the water of old latex paint, so I recommend disposing of old paint after 2-3 years even if it seems good. The only reason to keep old paint is for touch up painting.
Dispose of the old paint by pouring it over rubbish or trash and let the paint dry. Then leave the paint cans open with the lids off and let the cans dry too.
Dry paint is an acceptable waste but liquid paint is a hazardous material for disposal. Dispose of the dry paint and cans and the trash service is an acceptable disposal method. Follow the same disposal method for oil-based paints.
To get rid of large quantities of paint, please contact your City Hall for their preferred method of disposing of hazardous waste.