Do old forms perform less?

I am a web designer and one of my clients has received lately some negative comments from customers when completing the registration form included on her wordpress site. As the Jorfrom form is already 3 years old, I was wondering if it could be less suitable because of the many WP updates made since.

No, that shouldn't be the case since our forms are following all changes and updates across platforms. 

Could you tell us more about the issues your customer saw?