Looking at leaders of the swedish Social Democrats the picture is more complicated then a generational shift in leadership:

Hjalmar Branting
Leader for the Soc.Dems: 1907-1925
Prime minister: 1920, 1921-23, 1924-25
Class background: "High Bourgeoisie" ~ lower upper (father was a principal, mother was noble)
Notes: Same primary school as the king, spent his inheritance (a small fortune).

Per Albin Hansson
Leader for the Soc.Dems: 1925-1946
Prime minister: 1932-1936, 1936-1946
Class background: Working class (father was a brick layer, mother was a domestic aid)
Notes: Fabled for keeping his working class morals, according to one tale his wife returned govermental pencils after he died (he died while serving as prime minister).

Tage Erlander
Leader for the Soc.Dems: 1946-1969
Prime minister: 1946-1969
Class background: Middle (father was a teacher, mother probably homemaker)

Olof Palme
Leader for the Soc.Dems: 1969-1986
Prime minister: 1969-1976, 1982-1986
Class background: Upper (father was president of insurance company, mother noble)
Notes: Hated among much of the swedish upper class as a class traitor. Only swedish prime minister to have been murdered.

Ingvar Carlsson
Leader for the Soc.Dems: 1986-1996
Prime minister: 1986-1991, 1994-1996
Class background: Working class (single mother, textile worker)

Göran Persson
Leader for the Soc.Dems: 1996-2007
Prime minister: 1996-2006
Class background: Working class (father, construction worker)
Notes: Has made a much noted class journey and built his own mansion. Now well-paid consultant for private enterprise.

Mona Sahlin
Leader for the Soc.Dems: 2007-
Prime minister: Not yet
Class background: Middle class

Of course, this is looking only at the person at the top, a thorough study would include at least the executive council of the soc. dem. party, but this comment has taken enough time as it is.

Sweden's finest (and perhaps only) collaborative, leftist e-newspaper Synapze.se