Unlike the other characters who were maturing into adulthood, it seems to me that Ross was portrayed as more of a 'grown-up' back when the show started
I'm aware that most of the main characters were victims of 'Flanderization' in the later seasons of the show. The way I see it, this often came at the expense of their emotional intelligence. As in: Phoebe was aloof and blunt, but didn't come across as inconsiderate or mean; Joey was never the sharpest tool in the shed, but was capable of giving sound relationship advice to the guys. Yet, ultimately, they just of became more exaggerated versions of the first draft, and were at least able to achieve some sort of professional and/or personal stability they didn't have when we first met them.
So, all things considered, I feel that Ross is the only character who 'regressed' fundamentally - which might have something to do with him starting the show as the most 'grown-up' of the bunch, already financially independent, earning enough to not need a roommate etc... He spent the entire first season dealing with the emotional toll of his divorce, the fear of becoming a father, and the insecurity of opening himself up for a new love.
His character behaved like a reasonable adult and was given plenty of material that a viewer that was learning to navigate adulthood could relate to. But at some point it seems that the writers decided to bank on David Schwimer's talent for physical comedy, and Ross's standalone storylines grew gradually sillier (don't get me started on the tanning session!). Instead of the original awkwardness to ask a woman he liked on a date, he was always putting his foot in his mouth, misreading the signs and making the worst possible move. He was basically a goof with zero social skills.
Anyway, I just wanted to share this view because the running arc of "Friends" is about maturing into adulthood, and Ross's journey is the only one that makes me feel the adult was regressing into adolescence.