Cosmonauts Aleksandr Volkov and Sergei Krikalev were aboard Space Station Mir during the dissolution of the USSR in 1991. When they left Earth, they were Soviets. When they returned to Earth, they were citizens of the Russian Federation, and remain the only two space travelers whose citizenship changed while they were away from their planet.
A total of 112 cosmonauts have traveled in space, 77 from Russia and 35 from other former-Soviet republics. Only 3 out of the 112 were female.
Since 1951, precisely 268 cosmonauts have been selected for training in 18 rounds of candidate groups. In other words, far more than half of the cosmonauts selected have never actually flown missions. 156 vs. 112.
Why? Quite a few were unremarkably grounded due to medical reasons over the years, and still others participated in simulations or experiments, but simply never flew because there weren't enough projects to go around. Four were released due to "disciplinary reasons" involving alcohol, and a fair smattering disappeared after "personal" differences. A few died in air crash or car accidents, one was injured from a parachute jump, and another drowned during survival training. One was invited to retire due to a weight problem. The worst case, claiming to be in a "crisis of soul" after suffering depression and alcoholism, stepped in front of a train.
Mayhap the weirdest washout of all time, for any space program? Valentin Yershov was selected as a cosmonaut for the Soviet Union, but then locked out from training by TsPK ("Star City") director G.T. Beregovoy, because Yershov refused to become a member of the Communist Party.
Soviet Lunar Lander
The majority of stand-bys were so because of programs that simply never happened, such as the planned Soviet Lunar Landing, the un-realized all-female Soyuz mission, the not-built TKS spacecraft and Almaz space station, and the one-time-launched Buran shuttle orbiter.
Cosmonaut with the worst luck ever? Probably Pyotr Koldin, who was selected five times for missions, and grounded each time, due to project cancellations, re-assignment to the backup crew, or to make more room in the capsule so that spacesuits could be worn by the remaining two crew members.
Two science cosmonauts were named "Mars". Seriously. Their surnames were Rafikov and Fatkhullin.
The latest round of five Russian cosmonauts were chosen in December of 2010 as the RKKE-18 group. The youngest person now actively training for space travel, Syvatoslav Morosov, was born in August, 1985. The year "Back to the Future" and "The Breakfast Club" were released.
NASA plans to select new astronauts in 2012... wonder if they will select anyone younger?!