This has come up before, and sadly it's the case for me too
Of course something strange might happen, both might pull out, and Johnson might carry on until the next election.
If Starmer wins in a year or two, it will be a temporary state of affairs though, as he's older than Johnson.
Just going back over a few recent PMs:
Harold Wilson, born 1916, PM initially in 1964 (48)
Edward Heath, born 1916, PM in 1970 (53)
James Callaghan, born 1912, PM in 1976 (64)
Margaret Thatcher, born 1925, PM in 1979 (53)
John Major, born 1943, PM in 1990 (47)
So, someone born in 1917 would not have had a PM younger than them until they were 61 or 62. And someone born in 1926 would not have had a PM younger than them until they were 64. Lucky them...