Training advice needed!!!

Hello - My 14 month old Black Lab will not come, when called, when on a walk. She comes at the end for her lead to be put on and doesn't run off. But the problem comes when there is someone else in the park. She will run upto them and despite calling and whistling her she does not come back. You have to run upto her and try and get her back on the lead as she is bouncing around. Most people don't mind, she is just saying hello. She's the sweetest thing in the world and just gets so excited seeing people/dogs/bikes - you name it, she loves it. But it is a problem when there's an elderly lady or someone who doesn't like dogs.
I don't want to have to walk her on the lead all the time, she loves running around free - and i love watching her run free. But I am desperate to get it sorted.
Is it just that she is a baby and going through the 'teenage' stage? She learnt all the basics sooo quickly. By 9 weeks old she was doing most tricks. She knows to come when we whistle when she's in the garden/another room, but she does have that 'i know what you want me to do, but im gonna push my luck and do the opposite' look in her eye sometimes.

Any advice would be greatfully appreciated! She is 95% perfect - just need to fix this small but crucial factor.

Thanks guys.

Mary + Ruby