is it possible for your penis to grow after puberty? 

Men are often concerned about the adequacy of their genitals. Concerns over penis size and a desire for a larger penis are common in the male population. Many men wonder if it is possible for their penis to grow after puberty. Sometimes it can, but most often, it cannot.
The male penis normally starts to grow at ages 9 to 13 and completes its growth by 19 to 22 years. Most boys finish growing somewhere around age 16 or 17. Whether the penis has completed its growth during puberty determines how much residual growth is possible. Certain conditions do allow for penis enhancement. Hypogonadism, or testosterone deficiency, can actually create a Micropenis––a penis less than 4.5" in length when erect or stretched. Hypogonadism is defined as total testosterone of less than 300 ng/dl or free testosterone of less than 50 pg/ml in adult men. The effects occur mostly during puberty.

What it does mean is that early administration of androgen to prepubertal male individuals with low serum or salivary testosterone [Rilling JK, 1996] can result in a larger penis or phallus in adulthood [Baskin LS, 1997].
Men who have large penises show a rapid conversion of free T to DHT as measured by the T/DHT ratio in their blood. Rapid conversion is a genetic trait indicated by amplified maleness early in life: increased body and beard hair, increased musculature, early male pattern baldness, short height (<5'8"), and greater penile girth (>3.5" around) with length (>7"). A very strong sex drive, increased masturbation frequency, and early puberty are also associated with higher testosterone levels.
Research indicates that even in an adult male, penis growth is still possible with topical hormone application, causing stimulation of the androgen receptors in the corpus cavernosa, the blood-filled cavities along the shaft of the male penis.