Well i introuduced the english hackamore to 1 of my horses when he was about....... 18yrs old after being bitted his whole life..... was a very easy transition for him , and now being about 21 hes still going well in it.
My other horse however didnt really like the english hack, it was a bit to much for him. I tried it on him at about age 19-20yrs old after also being bitted his whole life. He had teeth problems at the time so needed a bitless type of bridle. I ended up getting a cross over bitless which works well for him, so i never went back to a bitted bridle.
The one thing i do caution greatly when choosing a hackamore, is its severety, in particular in a situation if your horse gets a fright. As its the riders natural reaction to ' snatch ' back on the reins, creating a quick jerking motion. In a severe hackamore this action could very well cause alot of damage without you intending it to, altho that is more related to mechanical hackamores. Not so much bosals, crossovers or ropehalter types.
What made you want to try out a hack? if i may ask.........