The ascidian Ciona intestinalis has a short life span and powerful regeneration capacities. The regeneration of the oral siphon (OS) involves wound healing, blastema formation, cell proliferation, and replacement of 8 oral pigment organs (OPO), the latter via differentiation and migration of stem/precursor cells from localized niches in the siphon. The restoration of OPO pattern during OS regeneration occurs with a high degree of accuracy through three successive cycles of amputation. It is shown here that oral siphons of the largest and oldest members of a wild Ciona population do not completely regenerate their siphons after amputation. The loss of regeneration capacity was accompanied by reduced cell proliferation. In contrast to arrested OS outgrowth, the stem/precursor cells responsible for OPO replacement "over-differentiate" after OS amputation in the oldest animals, the typical number of OPO is increased from 8 to 12-16, and malformed OPO are produced. Also in contrast to younger animals, the oldest animals of the population show arrested OPO development after two consecutive cycles of amputation and regeneration. We conclude that there is a size and age threshold in Ciona after which the regenerative capacity of the OS is compromised due to effects of aging on cell proliferation.