There are many lovely guineas waiting in rescues around the country for their Forever Home. Pet shops only sell babies that are a few weeks old. As fun as they are at this age, they really aren't suitable for small children to handle. Older guineas are usually more sedate, used to being handled and would enjoy sitting on your lap and being cuddled.
Reputable rescues will usually match guinea pigs with people and if you have an older guinea that has recently lost its partner, you are more likely to find a guinea of a similar age.
Pet shop guineas often come with "unwanted gifts" such as Lice, Mites, respiratory infections, fungal skin problems or surprise pregnancies.
Check out Guinea Pig Rehome where you can find a nationwide directory of guinea pig rescues.