I did this for my kids. I simply called Vanguard and asked them to send me the forms to open an IRA account. I filled out the forms for each of my children, had them sign the forms and sent them back with a check equaling the earned income for the year (up to $2000). I did not try to set these up as custodial accounts. As I recall the forms do not request date of birth, so the fund co. doesn't care.

This not only got them started early on retirement savings but also allowed them to file for a full refund of the income tax withheld from their part-time jobs.