what age do men no longer have precum ?

Precum is a fluid from his Cowper's glands. It does not have anything to do with age. Just like your arousal fluid does not have anything to do with age.

Precum cleans and lubricates his 'pipes'. Because he urinates and ejaculates from the same place, precum will push out all the left over urine, bacteria and old sperm to make it safe for the sperm in the next ejaculation when he gets excited. The Cowper's glands does not contain any sperm, nor does precum pass through sperm reservoirs.

The only way his precum can contain sperm, is if he recently ejaculated (masturbation, oral sex, hand job, intercourse) and the precum pushes this sperm out while it is still alive. After he urinated, all the left over sperm will be prectically dead.

This is not the advice you read or hear, but it is confirmed by various scientific studies of precum. It does not scare you into not having sex, so I suppose it does not make for good news or Internet.

Your problem with pushing the boundaries of safety like this is not precum, but ejaculation. When he gets excited, it is not always possible to predict when he will ejaculate. so rather be safe and use condoms. If you cannot afford to get pregnant right now, and want to be 100% safe, use the pill and condoms.

Best of luck, and stay safe!