45 day old SRAM rear derailleurs jumps gears

If the bike is new, it's probably cable stretch

All bike have a "settle in" period where the cables stretch, the bearings settle, the brake pads wear in, etc. However, this doesn't mean you have to take it to a bike shop. (most likely) all it needs is the cable to be tensioned. I would suggest taking it to a bike shop if you're unsure about derailleurs, as a tension adjustment usually costs $10 or less, and the mechanic may even be happy to show you how it's done.

If you choose to tension it yourself, here's how to do it: where the cable connects to your shifter, you'll find something called a barrel adjuster. A barrel adjust it a small piece you can rotate to adjust you cable tension. To tighten your tension (which is most likely what you need to do), turn the adjuster so that the top of the barrel moves away from the handle bar. Barrel adjusters are designed to adjust in quarter turns. Start with one quarter turn, check and see if the shifting improves, and repeat until either the shifting issue resolves itself, or about two turns. If the problem isn't solved within two full turns in either direction, something else is wrong with the derailleur. If that happens, return it to where it was and take it to the bike shop.

When the shifting skips over a cog on the cassette gears, this is because the derailleur hanger (or less often, the derailleur cage) is misaligned. That is, the small sacrificial tab of aluminium which is between the frame and the derailleur proper. This can happen easily on any bike if it is mishandled or the mech strikes a rock etc., the age of the bike isn't a big factor.

The solution is a derailleur alignment gauge, which a shop will have and can check the alignment for you quickly and easily. You could buy one yourself but they aren't the most common home tool because they're pricey and hopefully you need one only rarely.

Cable stretch would affect each shift equally, the cable loses tension and it is sluggish to change into every gear, because the start point of the indexing is out and the error carries over to all shifts equally.

Your problem is that it skips past just one cog completely, part way up the cassette, which is an indication of misalignment. Playing with the cable tension could bring a little improvement, but if you are not proficient with indexing adjustments, the misalignment will cause you added frustration, even if you follow the instructions correctly. In this case it's well worth checking the hanger alignment at the same time as any indexing adjustments.

I agree that your symptoms are not explained by insufficient cable tension. Alignment issues are a viable cause. From the rear of the bike, observe the derailleur cage while at both ends and the middle of cassette. The cage should be straight up and down--parallel to the cassette cogs--and the upper, guide pulley wheel should run directly underneath the cog of the selected gear. At the outside, small, high cog the guide pulley should run under the plane of the outside edge of the cog--just very slightly outside of running directly under it.

If the cage appears out of parallelism with the cassette cogs, a derailleur hangar alignment tool needs to be used to confirm and correct the alignment. In my experience, a cage out of alignment (the most common cause being a bent or twisted derailleur hangar) enough to cause poor shifting is quite obvious to an observer. I mention this to advise not getting hung up on a self debate if the cage is misaligned or is it in a normal position. If it appears straight up and down from a perspective behind the bike, it's likely in good alignment.

For a guide pulley wheel that doesn't run directly under the intended cog, manipulate the barrel adjuster to correct. Please note that it's normal finding that once the cage of the derailleur is lined up correctly at the limits and your hardware has good alignment, adjusting this guide pulley will not be necessary in most every cog as the alignment will be very close at this point. The barrel adjuster varies cable tension, and the tension effects derailleur movement--both in a fine tune cage position with the adjuster, and, of course, the shift. Setting the correct shift tension (indexing) takes precedent over cage position fine tuning. They go hand in hand but what I'm trying to say is to NOT be fiddling with the barrel adjuster in every cog up and down the cassette trying to get perfect cage alignment because that will mess with your indexing, and if things are normal you shouldn't be seeing misalignment of the cage with good shifting/indexing. Clockwise rotation of the adjuster causes a decrease in inner cable tension which has the effect of moving the guide pulley (and the entire cage) a few millimeters to the outside. Thus if you find your guide pulley running too far inside the correct cassette cog, clockwise rotation of the barrell adjuster (found on the shifter where the cable exits) will cause the guide pulley to move laterally to the outside. Conversely, counterclockwise rotation of the adjuster tightens cable tension and will move the derailleur cage with it's upper, guide pulley more inside, toward the next larger cog. This manipulation will correct a condition where the guide pulley is running outside of the plane of the correct cog's teeth. Again, though, if the indexing is correct, the cage will be well aligned with the correct cog.

Other considerations in your evaluation should be the external cable being fully seated within any cable stops you may have, proper routing of the inner cable at it's terminus through the rear derailleur and around the pinch bolt. Usually there is a groove it must be fully seated in and any washer like pieces of the pinch bolt should be secured in proper position so any leverage tabs are able to come into play if necessary. Because the bike is full suspension, the distance between the derailleur and the ground is variable making the effective length of the cabling vary. Check that your sag measurements are remaining consistent. Change in the spring rate will change the sag and thus also effect the effective cable distance which can change cable tension enough to effect shifting.

Here's a link to a GMBN video tutorial on setting up a SRAM mountain rear der. It covers well some things outlined above as well as a section on "B-screw" adjustment which changes the running distance between the cassette and the upper, guide pulley wheel. Incorrect B-screw adjustment can affect shift performance and would be a focus of my investigation of misshift symptoms such as you describe.