Are you referring to a rating system? Or maybe his 12th regeneration? If you are referring to the ladder, the reason Time Lords can regenerate 12 times is because after they pass The Time Academy, they receive a 12 count regeneration cycle and a Tardis.
Doctor Jeep was created in 1990-12.
Carry On Doctor was created in 1967-12.
Doctor Zhivago was released on 12/22/1965.
Doctor Who - 1963 Genesis of the Daleks Part Two 12-12 is rated/received certificates of: UK:PG
DR Who is not related in fiction or transcript in anyway to that of Dr Doolittle
Castrovalva - Doctor Who - happened on 1982-01-12.
Doctor in Charge ended on 1973-12-29.
Doctor Jones was created on 1997-12-08.
Robot - Doctor Who - happened on 1974-12-28.
Dragonfire - Doctor Who - happened on 1987-12-07.
Doctor Who - 1996 TV is rated/received certificates of: Australia:M Ireland:12 UK:12
Medium - 2005 Doctor's Orders 2-12 is rated/received certificates of: Netherlands:12 USA:TV-14