The fun (?) part to this is that the Astartes modification process totally fucks people up in the visual department. It's like getting your gender transed with all hormones (and some extra juices and organs) cranked up by the factor 40.000. The insane muscle growth that forms their posthuman physique? If you are unlucky that also affects your facial muscles and your tounge. Body hair? Might completly fall out due to the mix of juices and as a reaction to their artificial second skin being inserted. Meanwhile the body has not yet adapted to the acidic salvia but their healing properties are already on to combat it so their mouths and gums are chronically scarred/burned. Body might not have adjusted to some of the glands yet, causing a asymmetrical physique as well.
To make things worse they look absolutely starved as the modification process straight up sets their metabolism on fire and that takes a while to settle down as well - and all of this is not accounting for primarch DNA overwriting the original body in random spots: an originally dark haired Ultramarine Neophyte might suddenly grow patches of blonde hair, facial features shift, eye color can change, but all of this does not come at once but with time.
So the "fresher" you are coming out of the process, the more you look like a fucked up freak of nature furthest removed from humanity. It then normalises itself with time. Some have the luck of being in a legion where a proper skincare and wellness routine are appreciated - commonly inserted into times dedicated to meditation and prayer - like the Blood Angels, Thousand Sons, or Emperor's Children. Those get over that phase I can only call "puberty 2.0 insanity edition" somwhat well, the rest... well... has a somewhat rougher start.
So the younger the astartes, the older and totally fucked up he looks to the untrained eye.
