GREY HAIR ALONE MAKES NOT A THERA
Na tena thero hoti yen' assa palitam siro
Paripakko vayo tassa mogha jinno' ti vuccati.
He is not thereby an Elder (Thera)1merely because his head is grey.
He who is ripe only in years is called 'one grown old in vain'. 
HE IS A THERA WHO IS STAINLESS
Yamhi saccan ca dhammo ca ahimsa sannamo damo
Sa ve vanta malo dhiro thero iti pavuccati.
Only a wise man who comprehends the Four Noble Truths and the Dhamma, who is harmless and virtuous,
who restrains his senses and has rid himself of moral defilements is indeed called a Thera. 
One day, a group of bhikkhus came to pay respects to the Buddha. He knew that the time was ripe for those bhikkhus to attain Arahanthood. So he asked them whether they had seen a certain Elder as they came into the room. They replied that they did not see any such Elder but they only saw a young novice monk. Whereupon, the Buddha said, 'Bhikkhus! That person is not a novice monk. He is an Elder. I don't call one an Elder just because he is old. An Elder is one who comprehends the Four Noble Truths and does not harm others.'
- Thera— a term applied to those bhikkhus who have counted at least ten years in the Order from the date of their higher ordination. Thera, literally, means one who is firm or stable.