When are guinea pigs no longer babies?

Question
Hello,
It's me again, of the two shy 5-month old guinea pig girls. You recommended getting babies whenever possible for new owners, and now I am wondering out of curiosity, when are guinea pigs considered grown up? In the shop where we got them there were 6-week old babies, and apparently fully grown 5-month olds (who we got). Are they still considered young though? Yesterday one of them was, I believe according to all I've read, in heat! She was much more relaxed and friendly for a whole day, making funny noises, and with her sister following her around and sticking her nose in her rear. Both seemed quite frisky! Today they're back to being scared and running into their little house as soon as we come in the room.

Also, do you believe in spending lots of time with them to help overcome shyness, or to give them a little time on their own first? The shop-keeper said to leave them in the cage for 2 weeks, and pet them in there first, whereas my neighbor said to handle them as much as possible so they get used to it. I am also removing their house for short periods of time, as otherwise they seem to stay in there most of the time.

Thanks!

Answer
Hello Jenny,
I think Guinea Pigs are grown up by about 5 months I think, as when I wanted to breed they said to wait until they were 3-6 months old.
Yes handling is best to do asap! I found.
My rescue Guinea pig came in straight away and I held him all night, now he is a dream to catch and has never bitten or scratched me intentionally.
Yes I do that with my Piggies they have a little yellow hut but I only give that to them at night otherwise I feel it encourages them to hide away and not have fresh air. Just give them a tunnel or tube that is there for safety but not so much to hide, just something see through-able.

Megan.