The relevant CAP 2159 offence is possessing an indecent offence of a child. As you have learnt by now a child for that purpose is anyone under 18. Consent has nothing to do with it, so talking about “the age of consent” is meaningless.
I repeat my earlier questions, because they may determine whether you have committed an offence: what did you do with the image you were sent, did you save it anywhere? Did you delete the message which contained the image?