Chinese brushwork, acrylic on canvas
That phrase is taken from The Confucian Analects
The Master Confucius said, "At fifteen, I had my mind bent on learning. At thirty, I stood firm. At forty, I had no doubts. At fifty, I knew the decrees of Heaven. At sixty, my ear was an obedient organ for the reception of truth. At seventy, I could follow what my heart desired, without transgressing what was right."
Give me a quote as you wish. I'll translate into Chinese.