Jamieson, Fausset and Brown on Philemon 1:9

9. for love's sake--mine to thee, and (what ought to be) thine to Onesimus. Or, that Christian love of which thou showest so bright an example (Phm 7).

being such an one--Explain, Being such a one as thou knowest me to be, namely,

Paul--the founder of so many churches, and an apostle of Christ, and thy father in the faith.

the aged--a circumstance calculated to secure thy respect for anything I request.

and now also a prisoner of Jesus Christ--the strongest claim I have on thy regard: if for no other reason, at least in consideration of this, through commiseration gratify me.