Although I wouldn't prefer to term it as Shelf life, it's surely a sad truth.
The 'shelf life' of actors and actresses have nothing to do with the acting skills. Both genders have excellent and dedicated artists.
I believe the difference is due to the following 2 factors.
1. Fan base. Most commercial big budget movies cater to the fan bases. These fan bases are assured revenue sources. So the producers prefer to bank on those who attract larger fan bases. In this context, actress centric movies are less preferred. Actresses, even the top notch heroines of Bollywood have a slightly smaller fan base when compared to their male counterparts. This may be due to the lack of publicity or marketing techniques.
2. Type of roles they are given chance to play. The real magic happens when an artist essays a character that brings out their true potential. Most commercial cinemas don't have actresses in such character roles. They use the female lead only as a prop or a sidekick. Usually, especially in action movies, the heroines are supposed to have glamour more than the ability to get into the character they are essaying. So obviously as the heroines get old, their glamour drops and they are given lesser chances and slowly they die out.
But with more women centric scripts coming out, being appreciated in box office and actresses playing character roles rather than as props, I am quite confident that the 'shelf life' of such actresses will increase. Movies like Sarabjith of Aishwaria Rai, Queen done by Kangana Ranaut and ASK done by Nadia Moidu are examples of such movies which brought acclaim and praises for the female leads, who had supposedly expired their shelf life.
Let's see the world from different perspectives