What does 老大 mean in 少壮不努力, 老大徒伤悲? I was gaming with a friend at his house, and his grandfather told me this phrase which I don't understand.
The friend explained that 少壮不努力 means I don't study hard when I'm young, and he explained that 老大徒伤悲 means that a gang leader would be sad about me.
Obviously I know he's joking about 老大 being a gang leader, but he didn't want to tell me the real meaning, and I couldn't find what 老大 officially means. According to Wikipedia, 老大 could mean

The honorific title to the elder brother . 
Respect for the boss .
Respect for the leader . 
The name for the gang leader. 
Respectful title for chiefs and superiors . 
The honorific title for &quot;older people&quot;. 
Common name for &quot;the number one person&quot;.

What is the correct meaning for this context?
老大 here is not the noun 'leader/ boss'
It is an adjectives phrase
老 = old; 大 = grown (in age)
'老大' in '老大徒伤悲' means &quot;old&quot;
Detail break down of this expression:

少壮 - young and in your prime

不努力 - not work hard

老大 - grown and old

徒伤悲 - only have sadness


少壮不努力, 老大徒伤悲 means &quot;If you don't work hard when you are young, there will only be sadness for you when you get old&quot;