I can see why people would think he's "less than half" of Rarity's age, but if their ages were all left ambiguous on purpose. Spike has always been treated like an equal, as opposed to a child among adults.
Except for episodes where he's treated like a pet, an errand boy, or a narcoleptic infant. Plus who knows how rapidly or slowly immortal creatures mature? Rarity could be old and dementia addled before Spike hits dragon puberty. And that's of course assuming he isn't driven into madness when Rarity begins dating someone within her species.
Besides even if the ages were left ambiguous, Spike is only around 7 years assuming Twilight is a teenager like most claim and got her tramp stam-- sorry cutie mark around the age of 10. Rarity has not just a job but a decent reputation and a fair deal of experience, she's likely close to my age and in her early 20s. The exact age is left ambiguous but there's no ambiguity in the fact that Rarity is significantly older and Spike's crush is just the same stupid puppy love crush i had on the nurse in Silent Hill when i was 9 or so. Before she began bleeding uncontrollably from spontaneously opened wounds and went insane.