Tretinoin has been investigated the most out of all the topical retinoids for clinical effectiveness and safety, with over 20 randomised trials comparing it either to placebo or to other treatments.
Evidence of its effectiveness was first demonstrated in the 1980s. 
Certain genes, formed from sequences in our DNA, are sensitive to topical retinoids, which lead to changes in the function of the cell. They improve signs of ageing by: increasing cell division leading to increased thickness in the top layer of the skin (the epidermis); compaction of the outermost cells in the skin (stratum corneum); and production of glycosaminoglycans (complex chains of sugar molecules which attract water). 
Clinical trials looking at the effectiveness of tretinoin look at the various signs of ageing including fine wrinkles, coarse wrinkles, hyperpigmentation (brown marks), skin texture, laxity, and signs of ageing seen on skin samples under the microscope.
Three trials were published between 1988-1993 involving 55 patients between them; all showed statistically significant improvements with short-term (1-4 months) tretinoin use. 
As the short term studies showed continued improvements over time, five six-month trials were performed between 1989 and 1992 including 1,199 patients in total. All of these trials showed significant improvement in signs of ageing, even after six months. 
Since the six-month trials, longer-term trials have been performed, and consequently long term maintenance regimes have been recommended.