If we look at the Mouse Utopia experiment and try to fit the history of modern England into it
There could be an inflection point in 1921 when English population growth suddenly slowed - somewhat like the transition from phase B to C in the mouse utopia graph at the top of page 83
Then the plateau phase D - where births just replace deaths - was reached in the 1970s
Which perhaps (?) means the next phase would be the terminal death phase (among the native population - disregarding immigrants) with fewer births than deaths dwindling to zero live births and escalating median age until eventually all women are aged beyond the menopause.
Well, this isn't really comparing like with like! - and the whole picture is muddied by increasing medical capability and cossetting, which has radically reduced deaths from infectious disease (the main cause of mortality); and keeps infants and the elderly alive in circumstances which would previously have been fatal - but maybe gives us clues of what to look-out-for; assuming that the demise of Mouse Utopia was indeed substantially due to mutation accumulation ...