She's not actually older than them but people think that she looks older than them. Apparently, Sonakshi turned down Vettai's remake with Shahid upon hearing that Shahid joked about Sonakshi looking older than him. Feeling hurt, she turned down the offer.
She has also shown very keen interest in working with Ranbir Kapoor who did not joke about Sonakshi looking older, but nonetheless thinks so.
I think that styling herself in a different manner would actually make her look quite young and she already has this "innocent" look which works to her benefit. After all, she is working opposite Imran Khan in the sequel of Once Upon A Time in Mumbai.