Aveline
This is not an old person.
Had this problem with one DGS. It never used to be a problem but then I began to notice either less frequent washing or maybe infrequent change of clothes.
I tried, “Thanks for mowing the lawn/walking the dog/ picking the fruit etc. Would you like a shower now you're all sweaty.” He always accepted, but we didn’t see him that often, so it didn’t solve the problem.
Then when he started to come round for peace and quiet to revise/study, DH was more straightforward. He just told him he needed to shower more often in his teens then asked if he needed more tops so he had time for things to dry after washing.