If your career is still thriving and you're still pumpin' out good, relevant music without soundin' like the old guy tryin' to catch up... then by all means, a nikka could be over 40 and still do his thing. Not much of it will probably resonate with the Young Thug/Migos crowd, but any rapper that's been around since the 90s or so shouldn't aspire to compete with those type of muhfukkas anyway. At that point in one's career, their only true competition should be themselves. A Nas or Jay or Andre, Q-Tip, whoever... they don't need to try matching what the hot nikka of the month (no pun intended) is doing, or trying to rehash shyt they did years ago. Their aim should just be making quality music at this present time. It's not an age thing... even if listeners know a dude's on his way up in age, who the fukk's really gonna deny some shyt that sounds good to them cause of how old the artist is? 40 in 2014 ain't what 40 was in say 1992. If you were 40 and rappin' back then, yeah you'd probably look/sound old as shyt. 40 year olds now are nikkas that came up in rap's best eras and have grown up listening/doing it since they were kids.