Bakiyon ka pta nahi but Sonakshi really seems huge to be paired opposite someone like Shahid. It's more about how the couple would look on screen and whether audience will like the pair on screen or not. Sonakshi was recently paired with ARK in Kalank, so it's not like she isn't getting opportunities to star opposite the actor's if her age, I think it's more about how she looks with the actor she is paired with.
Someone gave an apt example about PC being paired with SRK and HR.
Kajol might be younger than HR but let's be honest would anyone like them paired opposite each other or Karishma for that matter. They belong to different generation of movies. Having said that I have to admit that yes, as a society we are programmed to imagine a male actor with a comparatively young female actor and not the other way around, which is really sad.
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