Unfortunately, once you're done growing, that's pretty much it. For someone in their later twenties, you're already not producing the regular pulsatile bursts of growth hormone needed to stimulate longitudinal growth, but even if an exogenous source was provided, the growth region of the bone known as the epiphyseal plate has already shut down, so you'd only grow laterally. If you get doses of growth hormone as an adult, you develop a condition known as acromegaly.