I guess it varies from person to person. I'm just basing it on my observations. With alot of people boyfriends/girlfriends seem to come and go but deep friendships tend to continue unless something majorly horrible happens.
You speak as someone who has never been in love. Ask a parent the first time they knew they could kill someone, anyone, if they had to, and they will tell you the first time they held their children. Ask a child the name of god, that child will say mother. Ask someone when they knew without doubt they would give their live for another, they will tell you the moment they realised they were in love.
It doesn't have to be a sexual thing. Love has different flavours. You can love people in different ways and taste the difference on your tongue. Most people feel it only for a child or a lover. Some, if lucky enough, will feel it for a friend close enough that in your heart you call them family. But make no mistake, to love someone, truly, without reservation, would make heaven itself tremble.