When does knee ache/pain become a serious concern? i feel a bit of heavy sensations around the area, just sort of aching a bit sometimes while walking

Before it does!: The best time to take things seriously (note: not worry!) is ASAP. Try a OTC knee brace for support and treat it carefully. If that does the trick, great! If not, have an exam and let results guide you to next steps. Don't worry but be caring toward yourself. Knees can be big problems as we age - especially if we've a weight issue. Best!