Douglas Gairdner, D.M., M.R.C.P.
"The Fate of the Foreskin, A Study of Circumcision"
British Medical Journal 1949, Dec. 24, 1949, vol 2, pages 1433-1437.
(entire text)

" ... the figures from which the diagram is constructed are therefore not precise, but they indicate with sufficient accuracy that the prepuce is non-retractable in four out of five normal males of 6 months and in half of normal males of 1 year. By 2 years about 20% and by 3 years about 10% of boys still have a non-retractable prepuce."

... "Of 200 uncircumcised boys aged 5-13 years from three different schools, 6% had a non-retractable prepuce; in a further 14% the prepuce could be only partially retracted."

Gairdner's figures are (in my experience) always the only ones which are used to show the frequency of adhesions and when they usually release during the first three years. I have so far only collected the following studies.

His 20% statistic among 200 boys aged 5-13 is never mentioned.

Kayaba H et al. Analysis of shape and retractibility of the prepuce in 603 Japanese boys. J Urol 156(5), November 1996, 1813-1815

Kikiros CS, Beasley SW, Woodward AA. - The response of phimosis to local steroid application. Pediatr Surg Int 1993; 8: 329-32.

Ludvigsson J, Fimosis hos 19-aringar - [Phimosis in 19-year-olds] Lakartidningen 1972 Dec 13;69(51):6028-9

Prof. Dr. med. Peter Rathert Dr. med. Stephan Roth, "Die Phimose", Dt. €rztebl. 89, Heft 48, 27. (November 1992) p. 28

The Kinsey Institute New Report on Sex. June Reinisch. Penguin (1990) p.340 - (no references given but they write) "By the age of three, the foreskin can be fully retracted in 80 to 90 percent of uncircumcised boys."