I looked at it, and there's actually no word there that directly translates to 'age difference'.
There is, however, the tag 'One-shota', which...well, yeah.
If it helps, the "おねショタ" (onee-shota) tag was user-added, so that just means some Pixiv user thought the scenario was rather suggestive, and I can't say I blame them.
They've purchased a Type-2 desk for the Secretary-ship and the Admiral.I've been wanting this!It's expensive. Is that still okay?Oh-ho.Is it okay if I call you "Oneechan" after all, Murakumo?Uh-huh.So, now I've got a favor to ask...So the cheeky fellow's learned to negotiate.