most babies will start to show interest in the spoon as soon as they notice it's there.... they're pretty fun to play with, and chewing on them has got to feel good on their gums!
HOWEVER, i'd warn you against giving them free reign over the feeding process, because once you lose control, it's hard for some parents to get it back, and then feeding becomes one really big mess with a lot of food going everywhere except for babies' mouths! i'd get into the habit of feeding baby ALL of the food, and then if they want to play with the food, you can hand it over and tell them "your turn" or some fun key words like that to make it okay for them to have the spoon.
slowly work your way into letting them feed themselves over a period of
months.... so that YOU can remain in control! maybe when you have food that is especially thick, you can give them the spoon for the last few bites that they can practice, but always use that key phrase so they know it's their turn.
somewhere between 18 months and two years old, they are really READY to self feed and it's a lot less messy and a lot less food gets wasted if you wait until then! oh, and when i say "self feed," i'm meaning foods they eat with a spoon.... they can feed themselves finger foods from the day they can pick up a cheerio and cram it into their mouths!
i know some parents talk about teaching their children independence and all that, but there's plenty of time for that! take advantage of the time you have to be in control, because it sure doesn't last long!