What are they saying instead of th, when did this start, and is it particular parts of Britain or particular social groups?

(Using f/v instead of th has long been a feature of some London dialects, and using t/d instead of th has long been a feature of some Irish dialects, and young children often use f/v instead of th, I think. Jocular references to "the youf" have been around in print for a while.)

EDIT: Of course there's also Jamaican pronunciation with t/d instead of th!

If you tell us what they say instead then I can probably give you some insight.

There's "bovvered" or there's "what da hell" or there's "fankyou"...

I can probably think of several more dialects where "th" is replaced with something else.

There's nothing sudden about it. Many regional (mostly city) accents have common mis-pronounciation. "th" being mis-pronounced as either "f" or "v" was extremely common in the area I grew up 20+ years ago, and I'm sure it predates me.

Why have Americans always been unable to pronounce the intervocalic T?

Bread and budder? WTF?

I'm an author; I love horseback writing?

I'm tired of wading for the flood?

I want to ride my modercycle?

It's so pervasive that we really ought to update our dictionaries to replace these, now phantom, "t"s with proper "d"s. Think of it as antipsychotics for a batshit crazy language.

I'm American. But this has always been annoying, along with most of the eN language.

To cast a cynical eye on the bright side; it seems both the "t" and "d" will soon be obsolete as the entire eN language is refined to tonal variations of our most common word/punctuation/expression/noise, "like".

I may be mistaken, but I believe there's evidence that new humans are forming a "like-gland". The sound will become as natural and necessary as that of frogs bleating* after rain.

*Yes, bleating, not bleeding.

I believe this is a sign of a hearing problem. Some children are unable to hear a strong difference between certain sounds so, to them, they are speaking the same as everyone else. It used to be that those children would be identified and given special help to allow them to hear better. I remember having eye and ear tests at school. Maybe this has stopped now?

The tragedy is that this is a huge dividing factor in the UK. In all the places I've worked the vast majority of people use the distinct "th" sounds. Use of "f" and "v" sounds is, to use the current terminology, a "red flag".