it depends on whether you mean a rubber one you can literally bend in half or a vulcanite bit. some people use rubber bendy ones when they first bit a youngster.
Well, some horses don't like metal bits, they'll be fussy in them, overbend, things like that whereas put them in a rubber, vulcanite or Happy Mouth and they suddenly become much happier (or vice versa, as the case might be!) I would always like to start a youngster in rubber/Happy Mouth, if at all possible; you can always go on to a metal bit later if needed. By their nature, these bits are usually mullen mouthed (though you can get covered other types) so less for horses to fuss with.