Earlier age gap was a matter to take an account of , however after the new UK student visa system called point based system came into effect now it becomes the duty of your academic sponsor to check your documents , if they issue you letter of acceptance then UK border agency will not investigate anything regarding the gap. Also a gap of up to 2 years is allowed for undergraduate and diploma students, plus for postgraduates its 8 years, also you need not worry about mentioning the gap since they ask you regarding the gap and all you need to do is to justify the gap. A one year gap is acceptable, plus the major reason for rejection of visa is insufficient funds and not the gap of education, so if you mention it just justify the reason, because your age would be reflected in the documents so think well and take the appropriate decision.