I do agility with all of my dogs. However, I can only underline what Hellen Louise posted. You must consider the physical developpement of puppy when starting this high impact sport.
The general rule is as follows:
1- between 12 weeks and 6 months: tunnels only.
2- At 6 months: He can start A-frame
3- between 6 months to a year: the dog can jump a hight no higher than his elbow;
4- At 9 months: The dog can start the long walk
5- At a year: he can start the weave poles;
he can also start the long jump.
6- between 12 months and 18 months, he can jump no higher than his shoulder;
7- as of 18 months, the sky his his limit...well not quite!
Jumping and climbing (the A-frame) can seriously affect the developpement of the joints. Remember that Beagles are prone to elbow dysplesia.
Weave poles affects the wrist and the spinal cord.
The reason for waiting to start the long walk is partially the climb, and also because there is a high risk of falls.
Having said that, do consider agility. Beagles love it and I am sure you will too. Ask Barking Mad! Oh, there is how ever one condition to doing agility with a Beagle, you have to post pics!