The real thing is not age but rather will and desire

Hadhrat Khalifatul Masih II ra says:

 "Thus, a person’s age is not important. In truth, it is the desire and resolve a person has through which he is able to attain nearness to God, and as a result of which, his efforts are able to surpass the youth of any generation. 

Thus, even though ostensibly the addressees of this sermon are those under the age of forty, nevertheless in truth I am also addressing those who, no matter what their age, are desirous of outstripping others in their service of the Faith. Such people are far more deserving of rewards than those who merely present the outward appearance of youth.

This is because the bodies of the young people are still capable of work but these (older) people's bodies have given up. Their example is like that of those poor who, despite having no wealth, do not wish to even entertain the thought of being left behind others when it comes to making financial contributions.

The Holy Qur’an mentions an incident whose details we find in ahadith; Once the Holy Prophet sa asked for monetary contributions. One companion got up and worked on a well and whatever he received as payment he gave to the Holy Prophet sa. Different people brought various contributions. Some had brought hundreds and some thousands. Among these was this companion who from his two hands  gave all that he had earned to the  Holy Prophet sa.  On seeing this the hypocrites laughed and said will the world be conquered with these scraps. Little did they know that in the sight of Allah these were greater than thousands - as he had nothing to his name. Yet he thought why should I remain behind others. He worked using his body. He laboured and whatever he earned he presented.  

The same is the state of these older people whose bodies may have given up but their hearts are young and they cannot tolerate being left behind anyone else in respect of spiritual rewards. In the eyes of Allah they are young, nay, even more deserving of spiritual reward than the young. In much the same way that the poor, despite giving small amounts, are more deserving of spiritual reward than many others who give multiple times more." 

(Mash’al-e-Raah Volume 1: p36)

urdu