Is it permissible for young men to grow their hair long, and what is its limit, to avoid the temptation of girls?

The basic ruling for a man growing his hair long is permissibility, as long as it does not lead to imitation of disbelievers or result in showing off. The proof for this is what Muslim narrated from Al-Bara' bin 'Azib concerning the description of the Prophet's hair (peace and blessings be upon him). As for the fear of fitna (strife/temptation) due to long hair, it should be avoided to block the door to evil and corruption. Beauty is a blessing that necessitates gratitude to Allah by adhering to His commands and avoiding His prohibitions.