I think part of the reason is that the role of Amanda has long been considered one of those roles that actresses do as a crowning achievement of a long career, sort of like Lear is for male classically-trained actors. It's been that way since Laurette Taylor's legendary performance in the original production. Taylor was in her early 60s, but looked much older as the result of years of alcoholism. Since then, few actresses younger than 60 have done the part in a major production. There also may be some vanity at play in this -- actresses younger than that tend to avoid roles that have them playing the mother of fully grown adult children as long as possible (wasn't Katherine Cornell still playing Juliet well into her 50s?).
I saw Jessica Tandy play Amanda in her 70s and Julie Harris play it in her late 60s, though their Toms and Lauras were in their 20s and 30s in both productions (I've also seen others, including Lange who starred in the misguided recent Leveaux revival). While the age discrepancies weren't distracting (and while it would be biologically possible for them to have children 40+ years their junior), obviously Williams originally intended Amanda to be a much younger woman when he wrote the play. I suppose Taylor's performance changed his mind and he certainly didn't object to her casting.
One thing that casting an older Amanda does is that it lends her grand tales of her youth a more rueful, sad and sometimes pathetic quality that adds a certain tragic dimension to the play. The notion of Taylor having been once the belle of the ball seemed somewhat unbelievable and fanciful, like she was an old woman caught up in delusions about her past. Having someone as youthful and still radiant-looking as Jessica Lange play the part (though she's technically more age- appropriate to the text) removes that quality entirely -- there's no doubt that she must have once been the most beautiful girl in the county who had multiple beaus show up at her porch every Sunday.
I'm not sure whether having a 50-ish actress play Amanda is of particular detriment or benefit to the overall play, but it certainly affects some of the subtext and the psychology underpinning the relationships in the play. For one thing, there's a greater sense of desperation in seeing a 70-something Amanda pleading with Tom to find a suitor for his sister before it's too late than a 50 year old one -- you get a real sense that an older Amanda doesn't think she'll be around much longer and wants to make sure Laura will be taken care of.
But, nevertheless I think the play can work regardless of the ages of the actors cast (apparently a 50-something Sally Field was quite superb as Amanda in DC a few years ago). As long as the director and cast capture the play's poetry, intensity and rich heartfelt nature (which were all missing from the Leveaux production) it's almost always a rewarding experience.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney