Lead paint was used up until 1976. It was used to provide brighter colors and to make the paint more durable. It has since been discovered that lead paint was a health hazard to children. Not that kids were eating paint chips, but the cast-off dust around windows and sills were being ingested by crawling toddlers causing learning disabilities.
There are various methods of handling the situation, from complete removal, which is very costly, or encapsulation of the lead paint, which refers to painting over lead surfaces with a specialized paint. Children are at extreme risk in a home with lead paint, but adults are hardly affected. If you are the landlord of a home that has lead paint in it, you must make full disclosure to potential renters. Sellers would need to also make full disclosure using a document called a lead-based paint disclosure statement that pertains to any home built prior to 1976. You must assume that any home built before then was at one point painted with a lead-based paint. There are kits you can buy at Home Depot or Lowes that let you test for lead-based paint. They are cheap and pretty accurate. It is worth checking into if children are involved. No doubt about it.