Circumcision is a tradition that begins with Prophet Abraham. There are many verses in the Old Testament about circumcision.
“This is my covenant, which you shall keep, between me and you and your seed after you. Every male child among you shall be circumcised. You shall circumcise the flesh of your foreskin, and it shall be a token of the covenant between me and you. He that is eight days old shall be circumcised among you. Every male child in your generations, he that is born in the house, or bought with money of any stranger, which is not of your seed. He that is born in your house and he that is bought with your money must be circumcised. My covenant shall be in your flesh for an everlasting covenant. The uncircumcised male child whose flesh of his foreskin is not circumcised, his soul shall be cut off from his people as he had broken my covenant.” (Genesis 17:10-14)
Researches made recently show that the chance of having uterine cancer is higher for the women who are married to uncircumcised men than the ones who are married to circumcised men. Other researches also show that circumcised men have a lower chance of having penile cancer than uncircumcised men.
However, being circumcised is not a must in order to become a Muslim.
It is a good thing that someone decides to be circumcised by free will, even if he is old. According to Old Testament, Prophet Abraham was circumcised when he was 90 years old (Genesis 17:24). But nobody should be put to pressure for this.