Retinol and its prescription cousin tretinoin (retinoic acid) have the strongest peer-reviewed evidence of any topical class for building dermal collagen, normalizing keratinocyte turnover, and reducing hyperpigmentation. The question of when to start is less about age and more about goal.
For prevention: late 20s or early 30s. The rationale is that dermal collagen begins subtle age-related decline in the late 20s. Starting a mild retinoid then — 0.3% retinol two to three times per week — supports collagen synthesis before visible changes appear. The evidence base for this preventive use is inferential, drawing on the well-established evidence for retinoid effects on collagen (Kafi et al., 2007), but the intervention is low-risk and the biology supports it.
For correction: any age. If you're already noticing fine lines, uneven texture, hyperpigmentation, or clogged pores, retinoids have strong evidence for improvement across all of the above. The strength should be titrated — start with 0.3% retinol or 0.025% tretinoin, work up as tolerated.
Considerations for perimenopause and beyond. Retinoids remain highly effective in perimenopausal and postmenopausal skin, but the barrier is often more compromised. Start low, buffer with moisturizer, and go slowly. Prescription tretinoin combined with adequate estradiol (through HRT or bioidenticals) has particularly good evidence for postmenopausal skin.
Never during pregnancy or nursing. All retinoids are contraindicated during pregnancy.
Sun protection is non-negotiable when using retinoids — they increase UV sensitivity.