Dear Shweta,
The other side of the coin
To look at it, there is always the other side of the coin. We have experienced instances where retired employees are no less enthusiastic than freshers, whether it be with the latest applications or imparting knowledge. Many freshers are seen as lacking knowledge in their academic line and are lukewarm in seeking and gaining the required information in their execution. This sort of mental state in them irks the seniors most, resulting in more workload for the seniors, including those re-engaged retirees. Of course, one cannot compare the utility of freshers with the retired; it is unfair. But age is only a number. It is misplaced to generalize that once retired, he/she becomes slow and lethargic. It is possible that persons suffering from certain ailments at retirement tend to be underprepared to continue with that much workload, but this is definitely not the majority. Backed by an enormous wealth of knowledge and work ethics, there is no reason to undermine their utility for the good of any establishment.
Compensation and retention
In the same breath, it’s unjust to pay them less. Many organizations extend services beyond superannuation with the same terms and conditions, of course, subject to one's policy providing for such an arrangement. I have come across persons who are retained by paying them more as well. Except for the disadvantage that he/she snatches employment opportunities from young job seekers who could be breadwinners for their families, I don’t find anything wrong in re-engaging them in cases of dire need, which is always there. Better not look down upon them just because they are available for retention. Why not more, since they would turn out more than expected (even unexpected things too, as they could visualize in advance more than you would, using their past experience)? Of course, the compensation package and methodology would vary from person to person depending on various factors.