Considering that Sarah having a son in her old age is explicitly presented as being “Huge incredible miracle beyond anything anybody could expect!!!”, I would say that you are supposed to understand that Sarah had a baby when she was 90. Similarly, I think it’s quite explicitly supposed to be unusual and amazingly miraculous that Elizabeth and Zechariah had a baby when they were both old.
Now, it is true that on occasion, women who have had lots of kids have continued having kids into their fifties, and that’s before fertility drugs or any kind of artificial insemination. Some people just have menopause reeeeeeally late.
But for a woman to have her first child when she’s gone through menopause and is old? That obviously requires divine intervention.
On the ages of the patriarchs, you have a lot more room to wonder. First off, it’s clear that some people were presented as living to be very old, and that humans live less long as they get further away from Eden’s time and God’s special graces given to Adam and Eve before they fell.
(FYI: Tolkien plays with this in the ages of his Numenoreans, if you read the appendices to Lord of the Rings or the Silmarillion. That is my geek moment of the day.)
However, there have always been plenty of commentators who taught that these ages are partly symbolic. Since number symbolism is really packed into Jewish writings and into the Bible, this is plausible.
Others have theorized that they also represent partly represent other calendar and year-numbering systems. This may well be, as some ancient civilizations did things like count harvests, and there is more than one kind of harvest in a year. But I haven’t seen anything really solid about this, and dividing Methusaleh’s age by three would still leave a pretty big number!
Frankly, once you say, “Oh, yeah, the Creator of the universe wants to tell humans stuff and make them part of His family, and He does not want them to forget what He tells them even after thousands of years,” you stop worrying so much about miracles being implausible. They are all signs, and God has the entire universe’s control panel to work with. He knows how to get our attention, because He made us. God knows every physical and biological workaround. Nothing is impossible with God.