Robots can't be blood siblings because they don't run on blood* , and you can't really tell their age from the official artwork because the official artwork is heavily stylized and as far as I know you have few other characters to compare them to.
Court Dragon
We... actually do have a few adult (or close to it) characters to compare them to, in the same art style, such as Kazuma and Momorin. In sprites, character portraits, and full art from the endings, the two robots appear noticeably younger than the adults — Quote moreso.
Their chronological age is uncertain, but probably not much older than 10 — they likely were made specifically for their mission.
I apologize for not putting this in the overall Cave Story topic, but I wasn't sure there was one, what the etiquette was on bumping/adding to old topics, and... pies.
crazy and proud of it
What I want to know is, what do robots use panties for?
Like, how would Quote and Curly even do it? Is there, like, a serial ATA port down there or something?
Or Fetish Fuel, depending on which way you lean on these issues. First off, as two robots of the same type created for the same purpose, presumably by the same person, they're essentially siblings. Then, there's the fact that they both appear to be physically underage — according to official art, Quote looks 13 at the absolute oldest and probably closer to 10-12, while Curly looks to be somewhere in the 13-16 range.