Then you can just block him. The more you contact him, the more he thinks about you. If you say you’re not interested and give a reason, he might try to argue or build resentment over being told he’s too old. If he’s creepy, safest option is to disappear from his life and be as invisible as possible. If you want to give him some courtesy, “I’m not interested” is all the info he needs.
When I first started blocking people, I blocked the number to be sure I wouldn’t break no contact. Now I can trust myself not to open (in case read receipt glitches) or send texts. So I keep them unblocked and ghost. This is so that I can screenshot death threats if any are sent, and show them to police.