It's my understanding that, in the first novel, Arthur McFly (George's father) briefly served in WW1 - before he was sent back home. He never even took his first gunshot.

Let's keep in mind that, when the first movie was filmed, there were no plans for a sequel. Therefore, Seamus and William haven't been thought of, yet.

William was born in 1885, and George was born in 1938. Therefore, William is only 53 years older than his grandson. WW1 had ended in 1918, which means that William would've been only 33 years old. In William had a son at 20, Arthur would've been only 13. There's no way one could mistake a 13-year-old boy for an adult man. Even at 15, which would make William 18 at his birth, would really be pushing it.

As I had Arthur be 17 in 1925, William would've 23 at the time of his mother. I gave him an older sister, by two years - meaning William would've been 21.

On a side note, here's why I made William's family the way I did.

William married Annette. I had Annette die in childbirth, to place Alice in the mother-like role. In 1925, here are the ages of the children: Alice was 19, Arthur was 17, and Agnes and Anita were twelve. That way Alice wouldn't be much older than her brother, but she would be old enough to have cared for Agnes and Anita. Alice would also be close in age to Arthur, so that they could be best friends - and have mutual friends. Of course, Alice also acted as Arthur's protecter. She also planned to be a nurse, which she did become.

I guess I did put a lot of thought into Alice. I figured that, in addition to having her look like Lea Thompson - there would also be the whole "mother" metaphor. I thought it would too far-fetched for William to marry someone who liked like her mother - so I gave him a wife who had the same body shape as her, but different hair and eye colour. That was also why I gave William brown eyes - so that his wife didn't have brown eyes, like his mother.

I guess I just figured that a good BTTF4 fic should have a Lorraine equivalent in it - and, of course, the "Mom, is that you?" scene.

For a twist, I figured that there should be a good Tannen - so I gave Jiff Tannen a twin sister, who was also Alice's best friend. I pretty much had to make them twins, as I decided to have their mother be born a Tannen - so that I could use the name Tiff Tannen (Gertrude was her middle name). For good measure, I also had Jill have a crush on "Huey Lewis". I initially didn't plan to make her be Jennifer's grandmother (not the one should would stay with, in Part One), but then a reviewer had suggested - so, I thought, "Why not?"