The typical silver-haired May was already done by Rosemary Harris in Sam Raimi’s trilogy and more recently Lily Tomlin brought a more badass elderly Aunt May to Spider-Man: Into the Spider-Verse.
Marisa Tomei’s younger May is refreshing and allowed her to differentiate herself from the many iterations.
When she was originally cast, she was willing to age up to fit the source material but they didn’t go with it.