Models are sort of backwardly compatible, but only if you save the toolbox to an older format.
If you right click ON THE TOOLBOX in Catalog (your reply to Dan above suggests you were looking within modelbuilder itself) you can SAVE AS. So if you had been creating a model in ArcGIS 10.3 but a client is using 10.1 then if you save the toolbox as 10.1 they will at least be able to see the model.
This does not necessarily mean the model will work:
I have found that sometime they may need to go into model builder and validate the model, this often clears the red cross which sometimes appears.
I built a model in 10.2.2 and eventually saved it in 10.3, that model is no longer even visible in a 10.2.2 environment. Can someone please confirm this is actually a bug as I am not finding any documentation. I would expect the model to error out when trying to open in the lower version, however to not even be visible seems very dangerous. I re-built an entire model, a lengthy one I may add, not realizing the issue and thinking I had lost my original model somehow.
In light of the silence: Boing.
Did you get any yeas or nays out of this?
We are running into similar problems with 10.3.1 models and 10.2.2 clients and prior to pulling all various iterations and documenting the jibbers out of the jabbers of it for a bug report I thought it would be nice to know if it is an already known issue.
The models are ho-hum clip,erase and merge models, nothing new in 10.3.
I mentioned this in another thread somewhere, but I was finding issues if I had three double-quotes as my comment block at the beginning of the script, but three single-quotes had no issue going between the version (10.2.x and 10.3.x) assuming all else is correct. Within the script had no bearing, just the first line.
As Dan mentioned in that thread, I should report it as a bug, but haven't since getting it to work was my isue and I've had to move on. But see if that is the issue in your case. I don't know how many times I rebuilt my Toolbox and Tools before I finally figured this out.
No, I was never able to find any documentation regarding the issue and the only responses I received were on this thread. After doing some more testing I believe there may have been an issue specific to that model as future tests worked fine creating, viewing, and opening models in 10.2 vs 10.3 and vice versa. Strange part is 2 models I had created that particular day/week are both not visible in 10.2 despite my future tests working fine. Of those two models only 1 was something important and that I had in production and which prompted me to post on here. Just something I had noticed and found odd. If I run into future issues I will surely continue researching and will update folks on here. Thanks for your reply.
I suppose this is more about making the point that backwards compatibility isn't documented, but a workaround would be to export your model as Python script, and create a new Python script tool in the earlier version, rather than a model. It should work the same, provided the Python functionality existed in the previous version.
At least in ArcGIS 10.5.1 all you need to do is to right click your toolbox in ArcCatalog and choose Save as and then pick the version you want the models to work with. But as described somewhere above you have to make sure you are not using any tools that aren't available in the previous version.