Disclaimer: this is written from my own personal point of view, derived from my own experience of being active in the kinky scene for eight and a half years. Your experience and perspective will undoubtedly vary.
In my experience among straight couples, it tends to be couples in similar ages that experiment with chastity together. From what is frequently posted and asked, the male who wishes to be locked tends to be the initiator and they seek advice on how to introduce the idea to their girlfriends/wives. For the girl to initiate chastity play, more often than not it occurs within already established kinky relationship versus dropping the idea of chastity to a completely vanilla male. For the experienced, the dominant wife/mistress/(insert title here) may even have chastity as a condition of their relationship.
Among gay men, the age gap is more prominent. For many, the desire to have that authoritative top/Master/dominant/etc leads them to go for older men that have the attitude, look, and gear they fetishize while they are single or simply uncollared. This leads to a large crowd of young men locked in service to older men. Their personal fetish can vary anywhere from simply loving sexual denial to being completely objectified as a sex or slave object/property. Chastity play among younger gay men is almost an antithesis to the nature of being a young gay male. Sex! Frequent sex, different partners, many partners...
The trend though, is evolving. From eight years ago when I physically entered the scene (years more of being aware of kink online), chastity has changed from being the odd fetish to being the coveted experience many want for real, but unwilling yet to surrender fully to their partner.
The one issue I've learned across the years is that reverse age keyholders, young keyholders topping older locked individuals is that the needs between them are different, sometimes too different for practical long term. Also, the expectancies of both the keyholder and locked person are similarly different. A young keyholder, while learning, is in the same boat as a young dominant first learning how to tie his or her first knots with rope. Their experience in domination, ability to exercise domination, their self control to restrain, are all in a very underdeveloped stage, relative to the older submissive seeking a dominant to take control.
The submissive may be expecting a certain style, competency, consistency, or any possible number of factors that may work towards or against their desire to submit to a younger dominant, chastity or otherwise. This is not to say that these dynamics do not exist. It is simply that the picture becomes more complicated when the roles are reversed with age being the prominent factor.
Mature men and women alike have already lived life for many years ahead of younger submissives. This life experience allows them to naturally take the role of being dominant in the manner that younger submissives find exceedingly attractive. Other variations of the dynamic exist and there are many online communities that support one another in their search and exploration of chastity among anyone and everyone. This subreddit is one of them. Go out there and have fun!