Supplant: To usurp the place of, especially through intrigue or underhanded tactics. To displace and substitute for (another): Replace: To put back into a former position or place. To take or fill the place of. To be or provide a substitute. To pay back or return; refund. They mean basically the same, but I would use "replaced" unless the younger man obtained the job by using underhanded tactics.

There is a difference. 'Supplant' can imply something negative - a replacement where the person was forced or removed by some trick. It can also imply that the person being replaced was inferior. If you told me a person was replaced, it could be because he retired or was ill. If he/she was supplanted, I might think that the person did something wrong or that there were some politics involved.