The first key as a senior player is to acknowledge that you will probably not overpower them. If you can accept this first fact, then the secondary fact is easier to do--Juniors hate slice and slow balls. They live in a world of Bash, they live to hit the ball hard and get a winner. Now add in the Juniors will probably be faster than you, the whole situation bends towards playing a more conservative, high percentage game.
The biggest keys will be first serve in and return in. The placement of a decent 80% first serve to their weaker side will gain you much benefit as Juniors tend to be momentum players, meaning they catch a lead, they hit harder while also feeding off each big shot. Serve the first serve to the weaker side and net people stay home to guard the alley. The other thing Juniors like to do is hit hard at the net person down the line, because normally in Juniors, the volley is not a strength. Now you're hitting first serves in and keeping the ball deep or short.
On the return, loops or slices to disrupt their groundstrokes will aid in your success. As we stated before, most Juniors do not volley well, so they won't poach. Hit it crosscourt deep and move in. By playing a style, they're not used to. The key is Play like a Senior player. This involves just playing solid doubles--hit first serves in, make first returns with chips and loops--high percentage stuff, then come in stealing time.
The last thing is not engage in baseline to baseline rallies--they will have spent more time on court hitting groundstrokes than you, they will be better at it. Bring them in to net, make them hit short balls, and dictate with your volley. If you have to rally with them because their groundstrokes are superior, use deep loops and chips. Finally, you can hit some to further disrupt their rhythmn.
The last caveat is to show no mercy because Juniors only respect you if you beat them. If you give them daylight, a few points, that could turn into games or a hot streak. Once they believe they can beat you, they will. For the most part, they start to think they can beat you, but in reality, they're not sure