We understand your concern. Here are the reasons that I think you might need to consider starting to learn programming with the new version:
First of all, you can program all Simatic controllers including S7 1200, 1500, 300, and 400 with version 13 but you can only program S7 300 and 400 PLCs with version 5.5.
Second, “TIA Portal” is the engineering platform that Siemens will develop in the future and you, as a professional PLC programmer, should work with this software for years to come. Also, there might be new S7-300 and 400 PLC modules in near future that won’t be supported by version 5.5.
All that being said, all of the PLC programs that you learn in the new version can easily be applied to STEP 7 V5.5 as well. I hope this help.