You meet a man and by default think he’s somewhere in his late twenties to early thirties but are then told then told (with proof) that he is actually 70 - would you be freaked out? Suspicious?
Yes, of course. Because it’s weird and seemingly impossible.
Are we in Middle Earth?
But you’d just deal with it, right? You wouldn’t ask too many questions and wouldn’t proclaim it to the hills, right? 
Not as far as I know, chap.
Your secret identity is safe with me.
Actually, if he seemed the kind of person who was proud of it, I’d talk about it with both him and others. It seems like a remarkable, yet mostly harmless, topic of conversation.
Jealous.
I’d think it was kind of sexy.
Most people who were cool when they were thirty, are still so, maybe even more, at 70. The only thing that has been added is wisdom, skills and knowledge.
Add to that good looks, and a lack of the cranky due to aches and pains and feeling vulnerable, and…well, more coolness.
I would want to see his portrait.
No, not really, it depends, I wouldn’t care. If I told anyone it would only be my ex and/or my mother.
I too would mostly be jealous.
A 40 year gap? That would be pretty amazing. But I seen a few folks that push it. Once met a mother daughter pair somewhere camping. Was thinking the mother was a bit on the young side for me. And her young teenage daughter definitely was (if not actually illegal). Both very good looking, cute as all get out, smart and funny. WTH were they or someone like them back when I WAS single?
Come to find out the daughter was in her late 30’s and mom was nearly 60 :eek:
Those two really lucked out in the gene department.
Apart from the impossibility of having a 70-year-old really passing for 30, I’d like to mention that older men should really think strongly about not wearing skintight Spandex bicycle outfits.
I don’t think I’d accept any proof. I guess I’m saying I’d refuse to believe it.
Isn’t the OP the same poster who asked recently how you would react if someone you knew grew elf ears or something like that?
OP, what’s going on?
ETA: http://boards.straightdope.com/sdmb/search.php?searchid=9667981
My aunt’s husband was in pretty darn good shape. Not sure that he could have passed for 30-ish, but he was 70-something and I watched him pick up the end of a parlor piano four feet high, seemingly effortlessly.
I’ve never met anyone with that kind of disparity in reality, although I do know a 68 year old who could easily pass for early 40’s, so that’s close. Except that I always do a mental double take when I remember his chronological age, I just pretty much think of him as his apparent age, and treat him accordingly. As long as he’s fit, I’ll ask him to grab the other end of the log to put on the fire same as I would any other person who looks like they could do it without injuring themselves.
His secret? Sunscreen every day before it was cool to wear sunscreen, and he’s never smoked. Eliminate those two things, and the skin doesn’t age nearly so quickly. Other than that, genetics have been kind to him, and he’s avoided hypertension and diabetes and other common illnesses of age. He’s not a diet and/or exercise freak or anything. He was a postal worker back when that meant a lot of walking, so perhaps that kept him well conditioned until retirement.
Occasionally as I pass him a cup of wine, I’ll joke that it’s Essence of Gelfling, though. 
What proof could this person offer?
Driver’s License? Fake
Birth Certificate? Belongs to someone else
Photographs? Photoshop
Any person that makes this claim is either an alien/wizard/demon/etc or just a conventional con man.
“Gelfling”? Is that a juvenile Genetically engineered life form?
My wife looks virtually the same today as she did the day I met her 30 years ago. She’s just one of those rare women who look good year after year, like Christie Brinkley.
No one thinks she’s 22 anymore, but no one believes she’s over 50. 31 is the average guess.
I suppose it’s possible for a person here and there to age better than anyone else. Why not?