No it's not too late at all. She's still a young dog really. No reason why she can't learn. A dog can't start learning agility until it is fully grown anyway because jumping and such is bad for growing bones and joints. A a BC, 9 months is usual age they have to reach before they start agility training. One of our former dogs, Monty, didn't start learning agility until he was about 2 and he did perfectly fine.

As for the weight fluctuations, I would seriously try not to let that happen. Weight going up and down all the time is quite stressful for the body, whether it be humans or dogs. If she isn't getting as much exercise as usual for whatever reason, her food intake needs to be lessened a little.
She weighs about the same as my BC who is now 13 coming up for 14. She filled out slightly as she got older as does happen with most dogs as with people...going up a few pounds and just looking a little more solid and shapely. She used to weigh less as a young dog but she was naturally more lanky then.

I agree that dogs who weren't introduced to the concept of play as youngsters might have a harder time taking it up when they are older. If there is anything she does enjoy, try incorporating a ball or other toy into it. Lots of exciting vibes from you will help to pursuade her. Even if she tries a half hearted attempt, be very pleased and show it and she'll soon learn that it is great and want to do it more and start putting more enthusiasm into it.
Don't automatically expect miracles though because all dogs are individuals...even BCs. We've had quite a number of BCs and they all have their own personalities. Our Jess and Mist who were mother and daughter were actually like chalk and cheese. Very different dogs. Some dogs might take the play concept on board and others might not...regardless of breed.

I must admit that mine were never the biggest players. They were more often likely to play with a toy when I was around either to get attention or because I was playing with them. They wouldn't play with each other when I was around. They'd only do it when they thought they were alone and if I wanted to watch, I'd have to do it secretly. Peeping around the curtain and they'd keep stopping and looking at the house and the windows and if they saw me looking out, they'd stop.
They did like playing fetch though and would do that until they dropped. Our Jess still does but I only through the ball a couple of yards now and only about 4 or 4 times with rest inbetween because running back and fourth won't be good for her old creaky joints now.