Birth control certainly was available many places in the US in 1955, in the form of condoms. Some places required a doctor’s prescription, but more places only required that you ask the pharmacist.
And despite the stereotypes about sexuality in that decade, there were a lot of folks who indulged in alternative sexual practices like oral and manual techniques, etc. which gave satisfaction yet avoided conception.
And married couples, even Catholics in the 50s, didn’t just keep having children every 11 months until they went infertile. My Catholic parents were married in 1952, yet I, their 4th child, wasn’t born until 1967! Is that a plot hole in reality?
(ETA: Yes, I know gestation is 9 months. But even parents trying to churn kids out as fast as possible have to wait a bit before starting up again.)
Why are you assuming he was born between 1957 and 1960? That’s an odd (and oddly specific) assumption to make. He had older siblings. But even if he hadn’t, there is nothing remotely strange about the idea that he was 17 in 1985.
Remember when he first shows the picture to Doc Brown he points out that his sister’s shirt has “Class of '83” on it, meaning she’d already graduated, so definitely older than him. And Marty’s siblings were disappearing from the picture he brought back first because they were impacted by his interaction with his parents first. If we assume that George graduated in 1955 and went to college, then married his high school sweetheart in, say, 1960 after graduating and getting a job, the ages make sense.
Marty was actually in his late 20s, and the oldest of the siblings. Like the actor playing him, Marty stopped growing at the age of 11. For the next 10 years on his birthday his parents would tell him he was turning 11. Finally at the age of 21 they were afraid he’d figure out his real age and started counting again from that point.
Yeah, I was thinking of one of my aunts, who, after two or three years with no kids, got a prescription for fertility drugs. And then it turned out that there wasn’t anything wrong with either of them after all, just luck, and bam, triplets. Fortunately they wanted a big family anyway, and went on to have two more (without pharmaceutical assistance) after the triplets.