In my experience, the average age at first marriage for women was actually in the early to mid-20s and therefore an unmarried woman of this age would not necessarily be considered an old maid, doomed to a barren, solitary life. Though there are plenty of examples of women who married for the first time aged 30+, this is closer to the age group I would label "old maid" or "spinster" since this is the smallest age group of women marrying for the first time. But it was certainly not unheard of. Consider the fact that childbirth took many female lives and left widowers with young children and no mother to take care of them. Often, a man might be pleased to take an older, never before married second wife to look after his children. It would mean he wouldn't have to take in her fatherless children from a previous marriage and since he already had children by his first wife, he would not have been as concerned about whether his second wife was still young enough to bare children or not. Of course, plenty of women then and now are able to start having children well into their 30s and even 40s but it does become less and less likely as time goes on.
But take, for example, my recent venture into studying the marriages of Butler County, Pennsylvania. I have ancestry there, mostly around the mid 19th century, and for the purpose of my family history writings, I wanted to get an idea of the average age that a woman would marry for the first time in this location during this time period, and also at what age the local law said a woman could marry without a parent's or guardian's consent. Unfortunately, the Pennsylvania County Marriages collection at FamilySearch.org only have marriage records for Butler County going back to 1885 but it was as close as I was going to get to the mid-1800s. Here are the results, I hope you find the stats as interesting as I do, though please keep in mind that different locations and different time periods may have different results, particularly regarding the age at which one can marry without consent of a parent or guardian needed. However, I would not be surprised if at least most of Pennsylvania had similar results, just based on my general experience doing genealogy research. If I had the time, I would do this for each available county in PA but for now, I sampled 300 records (out of about 630, basically the first half of folder 004811571) of the 1885-1886 records to get these stats.
84% of men were marrying for the first time, while 95% of women were marrying for the first time.
The average age of men at their first marriage was 26. Only 4% of men married under the age of 21 and therefore required parent or guardian consent. This is not unusual in a time when a husband was expected to support his wife and children so men were encouraged to wait until they had either steady work, set up their own shop, or established their own farm before they married and began having children. The youngest men married at 18 years old so there were no cases of men marrying under 18 at all. This suggests men were not able to marry under the age of 18 even with consent of a parent. The oldest age at which a man married for the first time was 47.
The average age of women at their first marriages was 23 and 30% of women married under the age of 21, requiring their parent's or guardian's consent. This means the majority of women certainly did not marry as teenagers, but that it wasn't unheard of, with 22% of women marrying under 20. The lowest age at first marriage for a woman was 15, suggesting girls under this age could not at all, even with consent. The highest age at which a woman first married was 49 years. Take that, spinsterhood!
Now let's look at some of the age differences between the bride and groom, since there also seems to be a misconception that it was very common for a teenage girl to be married off to a 30+ year old man. Again, not unheard of but also not the norm. In 88% of cases, the bride and groom were within an age difference of 10 or fewer years. In fact, in 11% of those, the bride was actually older than the groom! Of the remaining cases in which there was a higher age difference of 11 or more years, 30% of them had a teenage bride. The largest age different was 25 years, the groom being 47 and the bride 22.
Lastly, I did record some data from second (or third) marriages as well. The average age for a man at the time of a second (or third) marriage was 41, with the youngest age being 25 and the oldest 64. For a woman, the average was 40, with the youngest being 23 and the oldest 50. Divorce was certainly taboo but don't kid yourself that it never happened or that it was illegal - there were 4 cases where the groom remarrying had divorced his first wife and one case where the bride had divorced her first husband.
Today, the average age at first marriage for men across the U.S. is 29 and for women, it's 27. So while it's true that people tended to marry younger in the past, it was not so drastic as some people seem to think, with the averages instead being around 26 and 23 respectively (at least for Butler County, PA). I recall once hearing someone say that in the past, if one wasn't married by 18, they were "done", or had no hope of marrying. Hopefully, with these examples, I have helped to dispel these kinds of myths.