Maturation of the mammalian growth axis occurs when the production of key growth factors in the liver, namely IGF-I, become responsive to circulating growth hormone (GH) via its interaction with hepatic GH-receptors. While this process occurs around the time of birth in some mammals such as sheep it occurs well after birth in others including humans, mice and marsupials. To determine if nutrition, which influences growth and developmental rate, influences the timing of growth axis maturation in mammals we organized day 60 post-partum tammar wallaby pouch young into slow, normal and fast growth groups and measured the expression and circulating concentrations of key genes and hormones including GH, IGF-I/-II, GHR, IGFBP3 and IGFALS at 120 and 150 days post-partum. Slow young included those of primiparous mothers in their first reproductive year (n=7; maternal weight: 3.0 ± 0.4 kg), while normal young were those of multiparous females (n=16; maternal weight: 5.2 ± 0.5 kg). Fast young were fostered at day 60 post-partum to mothers at day 120 of lactation that produce a higher volume, energy dense milk, accelerating their growth and development (n=12; maternal weight: 5.1 ± 0.2 kg). Growth, development and maturation of GH/IGF-I axis components occurred earlier in fast growing young, which had significantly increased hepatic GHR, IGF1 and IGFALS expression, plasma IGF-I concentrations, and significantly decreased plasma GH concentrations compared to age-matched young in the normal and slow groups (p < 0.05, respectively). These data support the hypothesis that the timing of growth axis maturation depends largely on the growth rate and maturity of the young, which can be altered by changing their nutritional status.