(This Headcanon has been talked about a lot already but I'm adding something to it which I don't think anyone has mentioned yet from what I have seen)
Germany was not born a baby because he already had an existing body, this body was HRE. It is mentioned that Germany suffered many hardships and I think this was the remnants of HRE's bad health before he died e.g coughing blood and being restricted to his bed, (among other things to affect his health being obvious such as war). This is why Germany looks like HRE, he is not the HRE himself, but his body is the HRE hence 'Germany was a little older since he had an existing body already', this references not just the land but the personification of the country itself.
Also doesn't Prussia look like he is wearing a belt with crosses on it? This could be him protecting Germany from his demons (death and illness) which could also add to this theory of Germany suffering from illness, it was not uncommon many years ago to "ward off evil" (death and illness) with various religious methods such as using a cross. (Prussia has many crosses because he is being overprotective of his brother as always).
Germany does not suffer from illness anymore from what we have seen because his economy has become a lot better.
JE LEEST
Hetalia Headcanons
FanficThese are some headcanons I made up myself sorry if some of them do exist I probably haven't seen them, Hopefully my headcanons wont be ones you have seen before (it is what I am aiming for). My headcanons can differ from being normal to depressing...