Windows 10 PC not visible to other older version of Windows.

When you define your shares, add the Everyone account with whatever access level you want. Read, Read\Execute or Full Control then Share it.

For instance, to make the C: drive available to other network computers:

Add Everyone:

Open This PC
Right click on C: and select Properties
Click on the Security tab
Click the Edit button under Group or user names box
Click Add
In the Enter object names to select box, type everyone then click Check Names.
You should see Everyone
Click OKSelect Everyone then check the Permissions for Everyone boxes in the Allow column that you want to allow
Click OK and OK.

Share drive or folder:

Click on the Sharing tab
Click on Advanced Sharing
Click hare this folder
Give the share a name. I use names like Lenovo_C, Toshiba_C or HP_C, something that makes it easy for me to know what shares I'm looking at when viewing over the network.

Turn off password protected sharing:

Right click on Network and select Properties.
Click on Change advanced sharing settings at left.
Click the down arrow next to All Networks at the bottom.
Select Turn off password protected sharing.
Click the Save changes button.