I can't find any info on spacesuit-girl's age, on the official site, on Japanese Wikipedia, or in the magazine images we've seen. So we're left with just judging by her appearance, which as we all know in anime is not a perfect guide. But I'd expect her to be the same age as Erio, or maybe slightly younger.
In Nishida Asako's twitter, she mentions a new 1/24
plastic model of an
itasha (decorated car) featuring Erio.
Spoiler for Erio itasha images:
It's apparently a
VeilSide body on a Toyota Supra.
Nishida-san is also doing illustrations for a spin-off short story, to be written by the show's lead writer, Ayana Yunico, and possibly to be a BD/DVD extra. In her own twitter, Ayana-san praises Asako-neesan to the skies, says she loves her, snd that it is great that she is in this world. She also says she is fascinated by Meme and can't stop thinking of stories to write about her and Makoto.... This may not be one of them, but I do think that is Meme in front.