Agent M is an agent with the British Secret Service.
The "M" stands for Max, itself short for Maxine though she reminds people regularly (and curtly if they do not get it the first time) that she does not like being called Maxine. Max it is.
Her exact age is not specified but she is likely in her very early 30s at the most and more likely in her late 20's. This is based on the "years" of service she has already performed. In the period she has managed to create an impressive record of successes and a reputation for getting the job done. Not, however, that that seems to matter to her bosses for no matter how well she has done in the past, there is more than a bit of 'but can she really do the job.'
The job in question is largely the same - eliminating the enemies of the British government. In most cases it appears to be foreign problems but criminals sometimes rise to the occasion. To achieve the success she has done so far, she has trained to the point of exceptional skill.
Many times in the pages of her recorded adventures, mention is made of her incredible beauty and her dynamite shape, making her irresistible to her targets and getting them actually anxious for her to be near. Once there, her athleticism and her training in all forms of combat and hand-to-hand fighting make her a most deadly weapon. Add to that the fact that behind her gorgeous looks is a brain that can out-think most people and her usefulness to any mission is incredible.
Helping her on occasion is the equally beautiful and resourceful Virginia.