Sorry to link to The Guardian, but last year, a Dutch man's legal battle to self ID as 20 years younger than his actual age was rejected:
www.theguardian.com/world/2018/dec/03/dutch-court-rejects-emile-ratelband-request-20-years-younger
'In a written ruling on Monday, the court said Dutch law assigned rights and obligations based on age “such as the right to vote and the duty to attend school. If Mr Ratelband’s request was allowed, those age requirements would become meaningless.”
In a press statement, the court said: “Mr Ratelband is at liberty to feel 20 years younger than his real age and to act accordingly. But amending his date of birth would cause 20 years of records to vanish from the register of births, deaths, marriages and registered partnerships. This would have a variety of undesirable legal and societal implications.”
The court said it acknowledged “a trend in society for people to feel fit and healthy for longer, but did not regard that as a valid argument for amending a person’s date of birth”.
How is identifying as the opposite sex any different?
If Mr Ratelband’s request was allowed, those age requirements would become meaningless.”
And if men self Id as women then sex based rights become meaningless.
There's an apt use of 'trend' in this article too.