This is definitely the headcanon I’m most set upon.

I like to think he’s in the ballpark of 10 years older than Kiryu and Nishikiyama, which would put him around six years older than Majima, two years older than Adachi, and 13 years younger than Kazama, thus making him (approximately):

• 30 in Yakuza 0 (1988)
• 37 at the start of Yakuza 1 (1995)
• 47 through the rest of Yakuza 1 (2005)
• 48 in Yakuza 2 (2006)
• 51 in Yakuza 3/when he “dies” (2009)
• 61 in Yakuza: Like a Dragon & RGG 7 Gaiden (2019)
• 65 in Like a Dragon: Infinite Wealth (2024).

My reasoning for this is mostly that I don’t think he would have anywhere close to the same relationship with Kazama he has in canon if he were much closer in age to him. He is portrayed as an older brother figure to Kiryu, and certain RGGO interactions have Kazama asking Kashiwagi to think of him as his father figure. While I don’t view these dynamics as the be-all and end-all of their relationships to each other, especially considering this is the way a decent proportion of yakuza seem to view their superiors, I think it has to reflect at least somewhat on how old Kashiwagi is compared to both Kiryu and Kazama.

With regards to Majima, I don’t put as much thought into their actual relationship as characters (especially not until later in the RGG timeline), but something has to be said about Kashiwagi’s willingness to throw hands when Majima shows up to the office. I definitely don’t think he’s quite as young as Majima, but he speaks to him on the roof like a starving animal. Even then he has appearances to keep up, and he doesn’t let himself loose until they make it to Kamuro Castle. I always have to think about the fabled RGGO event where Sera mentions Kashiwagi mellowing out as he ages and what he might have meant by that. Was he any more riled up before 0? I don’t think he has any less than three or four years on Majima.

Coupling the last point with Adachi’s given, canon age of 59 in RGG7 and my unwillingness (for purely vibes reasons) to make them both the same age, I imagine Kashiwagi as being 1-2 years older, giving 1958 as his birth year.