at this place, 所长 is a noun which means the director of an Institute( research org / government entity, and in China it mostly stands for a government entity like a police station or tax administration). 老 means he has long credentials of the service as a director in this institute(so he is also possibly old). And by using 老 , respect is shown here. Some Cases: 1. 老 for showing respect and feeling being close to the one 他可是我的老上司. he is/was my elder boss.( might not be the current boss ) 他可是我的老朋友了. he is my old friend 2. 老 for showing repulsion 我恨这个老女人 . I hate this old women. Summary: Being aware of this typical usage of 老 is enough, it is not grammatical level but some kind of sense, so not very easy for a learner to use it in such detailed area. You can use other concise words which follow the definite language rule to express ur idea well. For example, if you want say my former boss, then use 我以前的老板; if you want to express that ur current boss is old, then use 我现在的老板，年纪有点大. In this way, you can make a Chinese understand you without any difficult, without any strange feeling, and you also have a definite grammar rule to guide urself.

why don't you just check 所长 in a dictionary. 老 is an adjective. 老means elder: This kind people are always respected and when you talk to this person who is in presence, it shows that you are close to this person.

"老所长" commonly address a boss who work at an institution or government entities.

非常正常的称呼 it's old所长 he~ 为了表示对长辈的尊敬 所以前面加上老字 还有一种情况是职位名称前加上老 代表以前是XX 但现在不是了 为了表示尊敬 所以称呼老XX 中国人很爱面子的 希望别人尊敬他 he~