I'm a body piercer and I work in a tattoo studio. There are no laws regarding the age at which children can be pierced, it's a considerable grey area (although it's generally accepted that genital/female nipple piercings are only carried out on over 18s).
I personally will not pierce babies. From a professional and personal perspective, I prefer informed consent from the person receiving the piercing (lot of Ps there!), not just the parent. The piercing is also at risk of growing out over time due to the very small size of a baby's earlobes.
I refuse to use guns for the same reasons outlined by previous posters (cannot be autoclave sterilised, causes blunt force trauma), and pierce solely with single use needles and titanium body jewellery as opposed to butterfly-backed studs made from cheap metal alloys, to reduce the chances of any allergic reaction.
For the record, neither of my children are pierced and nor will they be, until they are old enough to accept responsibility for any piercings and their aftercare, and have a full understanding of the piercing process. If other parents wish to have their childrens' ears pierced, that's up to them, but I do wish they'd make enquiries with professional body piercers instead of just going to places like Claire's, whose 'work' I end up fixing on a depressingly regular basis.