I knew the results would be that most posters think it's not unreasonable to be asked that. 🙄 In real life, EVERYone would be offended by this. There would just be different levels of how offended they'd be...
It's happened to me a few times, and frankly it has really pissed me off and offended me. You are right @WhatDoYouThink789 do NOT offer an over 60s - or pensioner discount to anyone! Put a sign up saying you offer them, and the onus can then be on people to ask for it.
I was offered a 'Silver Saver card' in a store this one time - (for over 60s,) and I was 46! Forty fucking six! I was like
What? I said 'why would I want an over 60s card? I'm in my mid 40s.' And I was like 
The (middle aged) woman serving said 'oops. I am not very good at telling peoples ages.' Just like this woman in the pharmacy who offered me a 'pensioners discount' when I was fucking 48! Again a middle aged woman. She said she is not very good at telling ages either.. If that's the case, why didn't these women think I was 35?!
I don't claim to be a babyface, but no fucking WAY did I look like I was in my 60s at 46-48 FFS!
I also had a woman - again middle aged! - offering me an over 65s discount several years ago when we went to a local visitor attraction. I was 55! I said 'umm, no thanks - I'm not over 65!' She just said 'I have to ask.' WHY? Why does she 'have to ask?' And does she 'ask' people who are 33?
Upshot is, don't assume people are pensioners or over 60. It's rude. And as I said, it's always been middle aged women with me. (early/mid 40s to around 60.) Men never do it to me. I have NEVER had a man ask if I want a pensioner discount! And no-one in their 30s or younger has asked either. Funny that!
Also, a few years ago (when I was early to mid 50s,) I met a woman who was walking her dog (when I was on a walk in the woods.) She told me she had just come off a cruise with her sister to celebrate her 60th and her sister's 65th birthdays. She asked me 'are you retired?' I was 54. 🙄I said 'yes I retired last year when I was 70.' She was like >>> 😕
I thought I would just go with the flow, and if these women make out they think I'm a pensioner/retired (when I'm in my late 50s,) I will humour them and say I'm in my 70s! I'm no longer going to let them see it's annoyed me. Again though, it was a woman between early/mid 40s and around 60.
(By the way, I have been mistaken for 5-8 years younger quite a few times, so I am pretty sure that I just pretty much look my age!)
!