This has always been a big thing for me, but it varies between siblings.
I have one brother who's 15, a sister who's 12 and a brother who is 5.
My 15 year old brother I've always seen as being 'young' - seeing him hit puberty was strange for me, because as a child he was my only little brother and I guess that stuck. When he was picking his GCSE/diploma options, it really hit home how old he was.
My sister is the total opposite of me, so I've always compared her to me at whatever age she's been.
My youngest brother got the opposite effect to his older brother - he's always been like a male clone of me, and so I remember what I was like at his age and consider him to be exactly the same, so treat him as his age rather than as I visualise young children in general.