Harrison Ford is a bit old for Uncle Ben. He is 72 , and he certainly looks quite old now.
Think about it really. If Peter is 16-18, then Richard Parker would have to probably be at least 18 years older. But say he's not, and he was 25 to 30 years old when Peter was born. Then how much older is Uncle Ben going to be that he's already 72 now? In his mid to late 50s? There would have to be a huge gap of about 25 to 30 years between Ben and Richard.
But even if there were, then how old was Ben and Richard's mother when she gave birth to Richard? If say, Ben was already 30 years old when Richard was born, she would probably have to be in her late 40s to early 50s at the very least. That doesn't really seem a very time to be having another child, especially after she already has a fully grown son. And why would she suddenly want another child then, or what would have made her wait so long between children for there to be a 25-30 year gap?
This is why it doesn't really make much sense for Ben and May to be in their 70s. Rosemary Harris was way too old for the part really.
And in fact, were Ben and May really that old in the comics? Or were they just depicted as such, because that was Steve Ditko's idea of what a couple in their 50s might look like, especially back in the 1960s? Even in TV shows or films back then, when people were shown to be in their 50s, they looked much older than what they might really look like.
A classic example is the Star Trek episode "The Deadly Years" in which Captain Kirk and some of his crew began rapidly aging. At one point, Captain Kirk was meant to be the equivalent of someone in his 50s or more, but he looked positively ancient, and way older than William Shatner looked when he was actually in his 50s, which would've been around the time of the first few Star Trek films.
I believe that it was just Ditko's idea of how older people should look, but while some do look old even in their 30s, there are many who don't look all that old in their 50s. Look at women like Diane Lane, Madeleine Stowe, Monica Belluci, Emma Thompson, among many others.
So I think an Aunt May and Uncle Ben who are in their 50s now, as opposed to being in their 50s when Peter was born is a far more suitable age, and their appearance should reflect it.
I'm not against casting an older actress if they look still relatively youthful for their age, like Meryl Streep, but I don't think Aunt May should ever look like Rosemary Harris again who could easily be mistaken for Peter's grandmother.