I think with adult garters handling isn't an issue, at least with mine anyways, they are very tame and will sit in my hand and don't feel any more difficult to handle than a ball python. I think the age is more a requirement because of their care, garters need to be fed and cage needs to be cleaned multiple times a week. Ball pythons for example it can be only one large rat every 3 weeks and as a result they will only defecate once every 3 weeks or so as well and forgetting to give them water or food for a long period isn't as detrimental as it is for a garter. I think BPs are much more tolerant of husbandry mistakes personally, some people may have different opinions than me however.