Don't, dude, don't make me think Flan is just a 5 year old when Chrino looks younger than her imo... D: Remi kinda looks like a child in EoSD but in IMP she looks like a mid-teen to me.
What about the 5 year difference? Well, considering ZUN is a jojofag, I wouldn't be surprised if he also likes to play with characters appearances as well as their ages. Like how Jotaro is 17 and looks 30 and Koichi is 18 and he looks like a little kid, so why couldn't Koishi?
So I guess it could be totally possible to Remilia to be "biologically" 5 years older than Flan and yet have a similar phenotype.
Certainly, the fact Touhou Vampires do "forever maintain and are trait-noted their earliest around teen youth", est. around pre-teen-to- earliest-and-early-teen teen 9-10-to-11-12-13,small grow could still possible in-between because there are partially corresponding variabilities is a Canonically Coherent fact which is also linked by hints and notes with " variable prudence and caution corresponding to her(Remilia's but also hinted Flandre's) variability about eventual increase and spikes about engagement, interest,curioexctiment but also eventual more reckless provocations and blow-away response"
All this without forgetting their outer development is showed by their Outer Primary Key Vampire-Youkai Trait...
Their Wings "Regular Unfurl/Furl" for Remilia, but for Flandre it would be a "more Personalized, Adapted, Atypical, Irregular such as her Atypical Wing-form", more clearly and bettef canon-quotes-and-images linked is the Development of their Wings, Wing-Span, Wing-Body Silouhette
This results in both a part of Youkai's personality maturity, one of maturity and one in development which is however mostly-separate from authority, role, attitude, knowledge and experience