Is going to the gym at the age of 16 a growth restriction for a boy?

No; provided you stick to sensible training methods within the hypertrophy/strength-endurance and stabilisation endurance ranges (speak to a biokineticist). Your number of repetitions should not drop below 8. You will become healthier and stronger more quickly than at any other time in your life as your growth hormone and natural testosterone are highest. Good luck.