You Can See The Difference.

I remember when I was over a friend’s house when I was probably in my late twenties or early thirties, we sitting around one weekend afternoon in the summer and one of the neighbors was there. We were talking about her son, if he was gay or not. He was showing feminine behavior (this was long before trans would have been thought of a possibility) and it was very obvious that his behavior was different from all his brothers.

'We had twin boys - but at 18 months old one wanted to become a girl': Father of transgender six-year-old on family's battle to accept Joe becoming Lily
Daily Mail
By Phoebe Jackson-Edwards
Published: 5 October 2015

A father of six-year-old twin boys – one of whom is now living as a girl – has revealed he first noticed differences between his sons when they were just 18 months old.

Matt Downes, 37, from St Albans, said he and his wife Rosie found that Olly would kick a ball around and buy Thomas The Tank Engine magazines, but Joe, who is now called Lily, opted for pink outfits and books about fairies.
[…]
After they were born, they were very close but from the age of 18 months, the parents started to see differences between their children.

Matt said: 'My earliest memory of it is at one and a half. My parents had them once a week from when they were born.

'Mum and dad used to take them down to the shops to pick a magazine. Olly would get Thomas The Tank Engine, or more "boy" magazines. She picked something pink with fairies.
'From the age of two she was drawn towards pink toys and babies. She asked for baby dolls and wanted girly fancy dress while Olly had a Batman outfit.'

And Matt explained that Lily’s ‘girlie’ behaviour continued when she went to nursery.

It is amazing to see how children gender identities develop at such an early age. Back when I was growing up feminine behavior was suppressed and we at a very early age learn to “act masculine.” Now we realize that by letting the child explore their gender identity greatly improves their quality of life.