Mei Tai Carriers generally have a 'body' piece of fabric, and four straps coming out of the body. Two of them go over your arms and tie around the 'upper and waist' part of your body and the other two go around your hips/waist (like a belt). There are no buckles in a true Mei Tai.
Depending on the brand, there are Mei Tai's that are appropriate for Newborns all the way through to toddlers. They have long straps so are generally suited to most size wearers.
It can get confusing looking at all the different names such as Mei Tai, Fly Tai, Didy Tai, Tai Tai etc ... but in general they are all types of Mei Tais (just different branding).
The exception to this is the Kokadi TaiTai which is considered a half buckle as it has a buckle around the waist and the rest of it has straps the same as the other MeiTais.
Please let me know if you have any questions, more than happy to help!