The  success of acne  therapy,  both  prescribed  and over-the-counter, makes any present-day study of the incidence of acne and its natural evolution virtually impossible.  However,  data obtained by this author's department  in  the period 1950-1975 provide useful information.
  The onset of acne is observed overwhelmingly in adolescence. A  1971 study showed that only 7 per  cent of cases developed even the mildest form of acne before the age of 12,  and 40  per cent first experienced acne after  they were  16.' Recently, however, there has been increasing awareness of late-onset acne, especially in females; the work just  cited only included  adolescents up to the age  of 20 years.  Acne may  occur before puberty. This is more common in females:  it has been reported that 3 per cent of female patients have clinical acne  before the onset of other obvious  features of puberty.2 Development of the condition before puberty is not usually associated with greater severity.
  The incidence of acne in adolescents has been found to vary  between  30 and  66 per cent  with  a peak incidence at  14-17 years in girls and at 16-19 years in boys.3,4 It is likely, however,  that all teenagers develop a 60 some, albeit at times mild, acne. Kligman describes 15 per cent of adolescents as having clinical  acne (acne major)  and  85 per cent  physiological acne  (acne minor).5 This is supported by the data of Burton et al,6 who examined  1,555 schoolchildren aged 8-18 years. Comedones were present in up to 80 per cent of the early teenagers and  in almost all by their mid-teens. Eighty-five  per cent of  the mid-teenagers  had  acne consisting of more than a few blackheads. In most cases it was  found that the severity of the  acne increased gradually, reaching a peak 3-5 years after the onset. In a small number of patients, however,  the maximum severity developed rather earlier.
  In this study a peak  incidence  was  demonstrated similar to that found by previous authors: at 14 years for girls and 16 years for boys. Thereafter,  the prevalence of the more severe types of acne increased in boys but decreased in girls.  In an even larger  series of 2,249 pupils  aged 12-20 years, an incidence  of 41 per cent was recorded, with a peak incidence of 61 per cent at 16 years.  The latter authors noted that comedonal  acne had a peak incidence at 12 years (54 per cent), papular acne at 16 years (58 per cent), and pustular acne (21 per cent) and nodular acne (12 per cent) between 16 and 20 years.1
  There is remarkably little information on the age at which the spontaneous regression of acne occurs. It has been suggested that this  happens in the late teens or early twenties.  Although this is undoubtedly true in most patients, a more detailed clinical  investigation of this problem is  required. The author has found that, in 4    ACNE 15 per cent of patients with clinical acne, the lesions persist in a form requiring active therapy until the age of 25, in a small percentage of subjects until the age of 40.2'7 Persistence  of lesions beyond the age  of 25 is probably more  common in  females,8  but this  may simply reflect their increased awareness of the condition. A perioral type of mild acne, consisting mainly of comedones and papules, has been described as existing in one-third  of American females between 20 and 50 years, and attributed predominantly to cosmetics.9 This cosmetic acne is by no means the entire explanation for persistent acne, however.
   A  small   number   of   patients,  in  particular females, have little or no trouble with acne until the late teens or early twenties when  the condition develops.9 Further studies are necessary to substantiate the observation that such patients often  have irregular periods and premenstrual exacerbation.  It has been shown that some of these patients with late-onset acne have elevated plasma androgens,10 but one recent report failed to confirm  this  (see  Chapter 9). Such subjects may simply, in terms  of  their sebaceous  follicles, be  experiencing a late  puberty.  Data are  not available  to support the suggestion that persistence of acne in a significant number of such subjects is  dependent upon their acne commencing in the late teens or early twenties.