Yes, your mother can apply for you. But each year, there are strict limits on how many people can come in.

You would be subject to the yearly quota of persons allowed from your country. Each country in the world has a certain quota established for it.

You would be in the second category for preference within your country. The top preference would be given to people whose family in the US are full citizens, rather than just green card holders.

The second level of preference, into which you fit, is for people like yourself who are children of green card holders and are unmarried, even if over 21, which you are.

After that, there are other categories with less preference than yours. But each level of preferences has a limit on the number of visas that can be given in a year.

So you have two limits to satisfy: the one for your country in general, and the one for your preference level as an unmarried child over 21.

Your mother can definitely sponsor your visa as you fall in the second category which is - family member of a U.S. citizen that includes , unmarried sons or daughters over the age of 21, married children of any age, and brothers and sisters of U.S. citizen petitioners 21 or older ( as per the USCIS ).Are you living inside or outside the US?

Your mother should first file I-130 ( immigrant petition) on your behalf. Once it gets approved , you can file in I-485 to adjust your status. This is possible only if you are presently in the US. The other process ,consular processing is the route to get green card if you are presently out of the US.

Will the person qualify under the "follow to join" category or can't since he/she is over 21 years old?