Our hearts know nothing of age, so I don't believe we ever reach a point in our lives where we can't fall in love. To love and be loved in return, is beautiful. Love is an emotion that, thankfully, cannot be fettered by mere age. It's boundless.
People have different views regarding the age which is considered to too old to fall in love. However, as much as we grow old, love doesn't. There are instances where old people, say aged 50+, meet and fall in love with each other. You stop loving when you die.
You can't be too old, but you can be too young. 18 (legally adult) is the age where you start to gain back the good judgement you lost as a teenager and didnt have as a child. 30 is where you've got it all back. You can't fall in love (in MY opinion) with someone over 20 years older or younger than you, as that is....well, screwed up.