Is it okay to download an older version of Photoshop?

I found an old Photoshop CS5 installer on internet archive and can use it for free (since it's so old)

For the record, and to dispel any misconceptions, it does not matter how old it is. CS5 is not free.

It's definitely no longer supported but you can only use it past its 30 day trial period if you already have a valid serial number due to purchasing the software in the past.

A safer option might be to use Adobe Express:

• https://www.adobe.com/express/
• https://helpx.adobe.com/express/system-requirements.html

Unfortunately, it sounds like your laptop is just a bit too old to run Photoshop on the Web:

• https://photoshop.adobe.com/
• https://helpx.adobe.com/photoshop/system-requirements-web.html

You not legally using it if it is not during the 30 day trial period.

If it is a cracked version, you should do a malware scan, as it may contain time bomb malware that will affect the system at some future date.

Lol correction: Affinity Photo and Clipstudio are not free but the trials are