<http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=+site:www.ssa.gov+women+minimum+retirement+age+europe+> just in case you wanted each specific country's requirements.
Initially, my WAG was that since unions promoted keeping people out of the labor (as with family wage and child labor laws in u.s.), then this was probably going on in europe, too, where they could exploit already existing gender ideologies, just as happened in the u.s.. This appears to be the case in Eastern European countries (below). however, a paper prepared for an int'l conference on social security, argued that:
In many countries of the world, the normal Social Security retirement age is lower (typically by five years) for women than it is for men -- a type of targeting that is never aimed at men. The lower retirement age enables women, generally married to older men, 6 to retire at about the same time their husbands do, a not insignificant benefit given their spouses' lower life expectancy. Some justify the lower retirement age for women as compensation for their double burden of paid work and family responsibilities. Whatever the original justification for the difference, it has been called into question in view of: (a) women's longer life expectancy, (b) the possibility of easing women's income needs by more work years, and (c) the need to control Social Security costs. http://www.issa.int/pdf/jeru98/theme3/3-2c.pdf
here were a couple of things I found re EE:
8. In the early years of the transformation process, gender issues were trumped by other apparently more immediate concerns. Initially, governments saw macroeconomic and political reforms as the most pressing. Through liberal early retirement and disability provisions, the existing social security schemes, prominently pension schemes, served to buffer soaring unemployment and hardship resulting from the economic transformation. Pension schemes thus contributed to the political stabilization of the new democracies during the early years of the transformation period.13 Later in the 1990s, fiscal concerns dominated the reform debates, creating a climate of belt-tightening, and a situation where government's capacity to compensate losers of reform measures was highly constrained. Under these circumstances, gender issues were only addressed in a limited way, if at all, or, as in the case of the retirement age, by imposing a disproportionate burden on women. http://www.oecd.org/dataoecd/40/47/2094610.pdf
and this suggests that there was a notion that women should be "rewarded" for motherhood regardless as to whether they were the primary caretaker:
In the Czech Republic, a woman's retirement age is still automatically reduced if she is a mother, regardless if she has been the primary caretaker or not. This is a reflection of a traditional perspective on women's role as "natural" caretaker and not an adequate measure to promote a more equal sharing of care responsibilities. http://www.oecd.org/dataoecd/40/47/2094610.pdf