Yes you can pray on behalf of your parents even though you are not the oldest. You can hire someone to pray as well if you choose to do so.
For a Wajib hajj, you can only perform it on behalf of one person. Also in order for you to perform the hajj you need to have performed the hajj on behalf of yourself in a previous year. You can give money to someone to do the niyaba as well.
There are many acts of sadaqa, one can recite Quran, give charity, do righteous deeds and pray for parents. These are all good.