Geriatric mothers were 26 and over when I trained!
This is an example of how medical definitions are often influenced by social standards of what is currently considered normal.
Right after World War II until the average age of marriage for women fell to very young ages (around 21 years of age), but women were still only having 2-3 children each (unlike women hundreds of years ago, who usually had around six children each). Through the 1970s, the average age of marriage crept up a bit ... but the average number of children that each woman was having went down, so that women were still having their last child at around 25 or 26 in most cases.
As a result, there is this very unusual period between the late 1940s and the beginning of the 1980s, where most births were to women in their early-to-mid 20s, and having a baby after about 30 was really unusual. Attitudes that women over 26 were "old" to be having a baby stem from this rather odd post-war fertility pattern.
It's not "traditional" in the true sense, by the way. The average Elizabethan woman circa 1600 (for example) married at around 25, and had around 6 children, with the last one probably being born when she was around 37 or so. The Elizabethans didn't think it was odd to have kids in your 30s, because almost all women who married would have had a few kids in their 30s.