There's a lot of misunderstanding about overlocking - I've seen it on lingerie and menswear from the '20s but the earliest I've seen it on ladieswear (apart from lingerie) is 1940s. It's common for professionally manufactured separates like skirts and tops to have it from the '50s. You do see some overlocked dresses in the '50s but it's not really until the '60s that it becomes more common, and from there it just gets more and more popular.
As Hollis mentioned, domestic overlockers became available in the '80s so home made garments earlier than this will be unlikely to have this finish, but professional dressmakers had them and there were a lot of them.
In short, it can help date a garment but it's not cut and dried.
Nicole