Is there an explanation why Marty McFly is a 17 year old senior, on Oct 25, 1985?

This thread gives me the perfect opportunity to link to comedian John Mullaney’s hilarious (and NSFW) analysis of the movie.

My parents got married in 1958. I was born in 1969. I do have older siblings, but my oldest sister wasn’t born until 1961. And we were a Catholic family, no methods of prevention allowed. Just because a couple is married doesn’t automatically mean they are instantly going to pop out all their babies in the first few years. Late-in-life kids do happen (and kept happening in my own mother’s case…I have three younger siblings.) There’s no inconsistency with Marty’s age.
Actually, what bothers me most about Marty’s time travel trip is that neither of his parents seem to recognize that their youngest son looks remarkably like the guy who was so vitally important to them getting together in the first place. Did Marty’s father never stop to think “Hey waitaminute, this kid looks suspiciously like the guy whom my wife was majorly crushing on just before we got together! And then he just disappeared – at least, I never saw him again…”

I did see an explanation for this somewhere.
(paraphrased) Marty was only in the past for a few days at most. It’s now 20+years later, they don’t clearly remember what Marty looked like. Since “their” Marty grew into looking like he does, it’s not the same as suddenly seeing Marty 20 years later. (paraphrased)

I could believe something like that, but it seems odd that they don’t have pictures from the prom showing Marty. If his dad ran across one of those he might start wondering.

This seems a bit silly as a discussion topic, but here’s my contribution…

My parents married in 1948. My older brother was born in 1950, I was born in 1953, my younger brother was born in 1958, and my younger sister was born in 1962. What’s the issue here? There were no miscarriages and every birth was planned (my mother always wanted a girl, so they tried four times).

Holy crap, my son is a dead ringer for that guy! Hang on; let me check another photo. Holy crap, he’s also a dead ringer for my great-grandfather, Seamus McFly!

Other than the posed formal pictures, there wouldn’t be any cameras around.

Like Prince Albert!

Then he notices that his wife is a dead ringer for great gramma Maggie, and decides never to think about the issue ever again.

I grew up in the 50s/60s. Large families were not uncommon, especially among Catholics, but most families were smaller. We had 3 kids, which was about typical. And we were Catholics.

Then he sees the pictures from the carnival of what looks like ZZ Top and quietly loses his mind.
Gets insanely rich from the resulting book.

In 1955? No camera phones, sure, but there’d likely be a floating photographer for the school paper and yearbook, and he’d get certainly of shots of the band. No reason to think there’d be a surviving closeup of Marty though.

People had Kodak Brownies and plenty of other cameras. It’s 1955, not 1855.

That’s the part I was thinking of. Did school papers use photos then? I graduated in 78 and the few photos used weren’t great quality by the time they wound up in print.

But it’s highly unlikely anyone would be carrying them on their person at the dance. Maybe stashed in the car but no impromptu selfies or group shots.

I was a bit not yet quite born in 1955 but a few years later is quite common for people to carry cameras to events like a prom. True, it’s more likely to have pictures taken at home or something formal at the prom that Marty wouldn’t appear in, but there’s the possibility some girl without a date wearing horn-rimmed glasses had a Brownie hanging from a strap around her neck.

True, but it’s unlikely that George & Lorraine would have copies of the pictures Ms. Horn-rim took. All they’d realistically still have in 1985 are whatever pictures are in the yearbook. Which are very unlikely to have a closeup of Marty.

The question I always wondered is Marty was so important to them meeting you would think they would have named their first son Marty rather than their second in the new timeline.

There are lots of people that happen to look like people from the past, and it’s common for people to joke that they’re time travelers. In general people don’t think that Eddie Murphie, Nick Cage, or Keanau Reeves actually traveled in time, they’re just considered an amusing coincidence. - YouTube

Now that would’ve been a great twist. Marty comes back to find his brother named Marty and he’s named Calvin.