Unless the mask was frequently removed and the skin cleaned, or the mask was somehow technologically able to absorb sloughed skin cells, hair, sweat, and oils, the person would die as an infant from cellulitis as bacteria/fungus would thrive in the moist conditions with plenty of dead tissue to consume. Even if the mask was placed at birth, when the skin is sterile (assuming a cesarean delivery, not a vaginal one that coats the baby with vaginal organisms) there would be enough penetration around the edges, via the nasal passages, etc that it would still infect. And even if the kid was on powerful antibiotics, the eventual accumulation of so much dead tissue would cause pressure ulcers on the flesh beneath, leading to a fatal inflammatory reaction. Especially during puberty when skin oil secretion really skyrockets, constant skin coverage would be a hygienic disaster.
But if the area is cleaned or the mask can deal with the skin itself, then persistent coverage would have several effects. The lack of UV exposure would lead to little pigmentation, unless there was some due to ethnicity. Although on occasion irritation causes an INCREASE in pigmentation. Callouses would form where the mask applied pressure, probably on the cheekbones, eyebrows, nose, and perhaps chin. Hair growth would be absent, either rubbed away or atrophied due to pressure in these areas.
Is the mask flexible/hinged? Does it allow for normal jaw movement? If it does, then speech, chewing, and teeth development may be relatively normal. If it is fairly rigid, then there would be atrophy of the jaw muscles and probably loss of teeth as the jaw would likely be small and the teeth crowded together, causing impaction. Being unable to chew means a liquid diet.
Presumably the mask grows with the child or is occasionally replaced. It would be pretty damaged and worn after 15 years of constant use. Its means of attachment would be the primary source of skin irritation (if it is mounted to studs placed in bone or somehow glued to the skin) or the straps used would, over the years, rub the underlying skin, limiting hair growth in that area and causing callous formation. Presumably the mask itself is fairly light so unless it is very rigid or tightly bound it shouldn't interfere with the underlying bone development much (other than the jaw if the mask hinders jaw movement).
If the mask can't be easily removed, isn't loose enough to scratch under, or thin enough to scratch through, then the poor kid is gonna have some hard times when there is an itch under the mask. They will also have some significant socialization issues if everyone they meet is also wearing a mask, they will be unable to understand facial cues (and probably develop a strong ability to read body language instead). Even if they see normal faces, having a mask on will prevent them from really developing a good facial language of their own as they will never get any feedback from any facial language they are attempting to use as an infant (unless the mask somehow can convey a smile, frown, etc). They will most likely adapt body language cues instead (assuming they are getting any type of normal socialization). Other kids will be a problem (if they are not wearing masks) and I imagine the masked kid will have a pretty isolated childhood since the mask would be a significant distinguishing feature.