They're Heisei. You can't just bend periods of time because something "feels" like it should belong to a different era, Toei/fans/both.
To be honest though, I don't think any of the terminology ever really works. I despise "New Gen"/"Old Gen" because they a) look stupid and b) don't and won't make sense after a given period of time. Kuuga is not new anymore. In 30 years, Double is going to be the old stuff that mom and dad watched while the kiddies have their Kamen Rider Biomechic Fusionator.
"Ishinomori Era" also has its problems. I mean look at the Kuuga thing... some people swear up, down and backwards that he planned the entire thing, plus Agito and 1/2 of Hibiki, when all he really did was sign off in 1998 on the idea that "yeah, let's do another Rider series" and some preliminary sketches which- trust me- are a faaaaaaaaaaar cry from what ended up as Kuuga. Some fans consider 1~ZX to be unique in the fact that they're made under Hirayama, but if you're going to divide things by producer then it gets really confusing.
Using the actual Japanese dating system is the most logical, aside from Toei generally not knowing where to put Shin, ZO & J. They're grouped with the older guys, probably cuz of Ishinomori and the gap between J and Kuuga. Fair enough, though it's still incorrect to call them Shôwa because, well, they're not. "Classic Riders" or something, okay. I mean hell, if you must have them appear in the Shôwa group photo, fine. But know the difference.
Thus I prefer to just say "the older Riders" and the "newer Riders" cuz honestly, Kuuga, Agito, etc. are feeling pretty old to me now. :laugh: