mysqltuner: "MySQL started within last 24 hours - recommendations may be inaccurate." Why?
It is a basic caution that a MySQL instance running under 24 hours (1 day) may not have seen the entire query workload and thus the recommendations might be affected.

Of course, 24 hours doesn't guarantee the entire workload either, as you might have scripts that run weekly or monthly.

On a separate note, I would caution you on taking the advice of mysqltuner too strictly. Specifically don't make any changes that you don't fully understand just because the script says to do so.