Here is some information you might find useful.
Since the latest Debrett's does include the "presumed heir" and his son for the Cavan earldom as Colin noted, I think there is hope for this peerage.
There are 4 viscountcies who each have only one heir 50+.
Montgomery of Alamein (cr. 1946). Current peer b. 1928, heir b. 1954
Norwich (cr. 1952). Current peer b. 1929, heir b. 1959
Stuart of Findhorn (cr. 1959). Current peer b. 1948, heir b. 1957
Tenby (cr. 1957). Current peer b. 1927, heir b. 1962.
Henry, I know your list was limited to only 1 heir, 50 yrs old or older, but if you expanded it to include 2 in the line of succession, 50+, there are several peerages which would fit that criteria. These peerages also face extinction in the near future. These peerages are as follows:
Marquess Camden (cr. 1812) Current peer b. 1930, those in succession b. 1965 and 1952. Also includes E Camden, Earl of Brecknock, Viscount Bayham and Baron Camden.
Earl Jellicoe (cr. 1925). Current peer b. 1950, those in succession b. 1953 and 1966.
Earl of Tankerville (cr. 1714) Current peer b. 1956, those in succession b. 1958 and 1961. Also includes Baron Ossulston.
Earl of Westmeath (cr. 1621) Current peer b. 1928, those in succession b. 1965 and 1966
Viscount Monck (cr. 1801) Current peer b. 1953, those in succession b. 1957 and 1961. Also includes Baron Monck
Also, I believe Viscount Craigavon's birth year is 1944 and there are no heirs to the Gough baronetcy.
I will try and post more information as I can.
Hope this helps.
Brooke