In 2013, the original Karate Kid turned the as the same age as Mr Miyagi in the original film (and doesn't that make you feel old). True, to a point, but it's a bit of an optical illusion
TRUE - Actor Pat Morita was born in 1932 and so was probably 51 while the 1984-released film was being made. Actor Ralph Macchio was born in 1961 and turned 51 in 2012.
HOWEVER - As often happens with Hollywood, teenagers are played by actors in their 20s. Ralph Macchio was 22 during filming, playing Daniel, a high school senior, so probably a 17 or 18 year old. Also a Hollywood tradition is casting actors who look older than they are as seniors. At the age of 11, Pat Miyagi was placed in an internment camp with other Japanese Americans. However, it is revealed that Mr Miyagi's wife died during childbirth in an internment camp and he later served in the US military, making the character at least 7 years older.
SO... Although Ralph Macchio was the same age in 2012-2013 that Pat Morita was in 1983, it won't be until at least 2024 until Daniel was as old as Mr Miyagi.