Where did it say they're considered old at age 17? I thought it was by the age of 25 or 30.
(For the following bit, keep in mind that I am not endorsing anything here. I mainly know about this because I had to do some research for a writing project.)
From what I know, the medical community seems to agree that the age of 16 is the earliest for girls to start safely delivering babies - with exception for girls who are on very early or very late developmental curves. Granted, they also say that its best to wait until they're in their early twenties due to their brains still being in development. But that hardly matters in a setting where people tend to keel over by the time they're 55. What matters in that case is that you start pumping out babies as soon as possible.
So, if this society's expectation is that people should be having babies by the age of 17, then that I could believe. But old? Not even with life spans that short.
There's a lot of cultural factors that can play a part as well. There are some periods in both the ancient and medieval world when the average age of marriage for women might have been fairly low, as low as 20. These periods are bookended by periods of history where the average would be more like 25, not too far off from most modern figures. There's no indication that for these periods, fertility or health was a factor, so the influence may have been cultural, though likely in terms of influx of under-16yo marriages.
That being said, even taking that into account, I don't think there is any known culture where women over the age of 20 are considered completely expired goods. It would simply be a hugely unrealistic waste, and it's unlikely that any arbitrary cultural trend of preference for younger women would be able to overturn that entirely. At worst, a woman past 25 years or so might be considered a "B-pick", pretty much the same as today with impressions of women over 30. And we may joke about them being old, but generally people would have been considered old only when they were incapable of working. Even in the ancient world, that would at least have been at worst around 50, but usually 60~70 same as today. This would be independent of the average life expectancy...a culture could consider someone old at 60 but have an average life expectancy of 55 for various reasons.