As you're young, you can get away with something along the lines of 'ooh aren't you funny/smiley/interesting/whatever 'compliment' works in the situation, you remind me of my grandpa. Have you got any grandkids? I bet they think you're a funny/smiley/interesting/etc old thing don't they! I just can't imagine being old like you and having grandchildren...'. Say this with a smile, and as if you're talking to somebody who's a bit old or dim or a toddler, just making conversation rather than a serious comment and carry straight on into a different conversation without giving them a chance to discuss any further. (So an 'anyway, have a nice day, thank you for shopping with us' type comment.)
Completely irrelevant if he is actually like your grandpa or funny/smiley/interesting/etc. But you will have been polite and made conversation with the customer without giving anything in particular away about yourself - and you will puncture the ego of any 35yr old or older that you use the line on - because they will be talking to you thinking that they are chatting you up and that you're a nice 17 year old girl that's responding back to them, they're thinking woohoo, maybe I'm in with a chance, only to discover that you think they are grandfather age and that you think they are like a grandad rather than a potential mate... Or if you think they're maybe in their 20s, switch grandad to dad.
But if they complain (unlikely, they're not going to want to advertise the fact that somebody thought they were old), so long as you're saying it like a nice compliment, then you can genuinely be surprised and say that you were trying to pay them a compliment. If they push it, you get to push back and say well why else would they be chatting to me about this and asking for my facebook details? And then if they push again or mention dates or meeting up or anything really - you get to say I'm sorry, but not only are they a customer but they're significantly older than I am, that's really yukky, why on earth would I be interested in dating somebody who is old enough to be my grandad, I'm just trying to be polite to a customer'.
The other great phrase to get out of situations where you don't want to say anything negative to people is 'I'm sorry, I think we must have been talking at cross purposes...' (and in this case could have followed it up with I'm friendly and polite because you're a customer but obviously it's inappropriate for us to be facebook friends).
It's a great phrase that has been really useful for me - I'm fairly polite and non-confrontational and it's a graceful phrase that doesn't assign blame or wrong to either side, lets both people's views be valid (even if inside you're thinking that they're being horrible or inappropriate or yucky or whatever) but moving on from that point they have to accept that you did not agree with that view, nor do you now and that they need to understand that you don't want to do/be/etc whatever they want, but that you will be doing what you said (so in this case, not being facebook friends with a customer).