Since 1950, the U.S. has elected 3 presidents younger than 50 -- JFK (43), Clinton (46), and Obama (47) -- and 2 more younger than 55 -- Carter (52) and George W (54). The Republican candidate has been a decade or more older than the Democratic candidate 7 times in the last 10 elections (including three times by 2+ decades). The Republican has been younger only once, by 3 years:
2012: Romney (65) +14 v. Obama (51)
2008: McCain (70) +23 v. Obama (47)
2004: Bush (58) -3 v. Kerry (61)
2000: Bush (54) +2 v. Gore (52)
1996: Dole (73) +23 v. Clinton (50)
1992: Bush (68) +22 v. Clinton (46)
1988: Bush (64) +9 v. Dukakis (55)
1984: Reagan (73) +17 v. Mondale (56)
1980: Reagan (69) +13 v. Carter (56)
1976: Ford (63) +11 v. Carter (52)
Hillary will be 69 on election day. That makes her the oldest Democratic candidate in a long time, but only a little older than the average (65.7) and median (66.5) for a Republican candidate.