At 26 years old, Mary-Kate Olsen isn't ready for marriage -- even though her 43-year-old beau Olivier Sarkozy is dying to put a ring out, sources reveal in the April 8 issue of Us Weekly.

The once-wed multimillionaire "wants to lock her down for life," an insider says of Sarkozy. "He calls her 'smart as a whip' and says she's mature for her age."

The actress-turned-designer began dating the French banker 10 months ago, and "she's making him wait," a source tells Us. Adds another, "Mary-Kate wants to give it more time. She's not in any rush."

For more on the couple's blossoming romance, watch this week's Hot Stuff video, as explained by Us Weekly entertainment director Ian Drew and special guest, Florida Georgia Line!

Also in the roundup: Sofia Vergara will use a gestational surrogate to have a baby with fiance Nick Loeb; why Hayden Panettiere is keeping her engagement to Wladimir Klitschko under wraps; and much more!