I am older as well and just started playing osrs again after a long break. I have been slowly working on getting most of my skills up to around lvl 30. That's around where the leveling slows down and things get a bit more grindy.
Also check out the optimal quest guide, I have been working my way through it and it's a great way to get exp and unlock new skills, areas, and features. The quests are also adorable and memorable, my household regularly makes reference to "One Small Favour". :)
Edit: Also, check out the Runelite client. It is a client to play rs in, and offers tons of quality of life fixes for the game. Some of my favourites are a graphics update making everything a bit prettier, and a weather plugin that gives dynamic weather. There are a ton of useful ones as well, for everything from inventory management to skill tracking.
I logged into my character for the first time in probably 13 years just a couple days ago. I knew my progress would not have been carried over into OSRS, since I had missed that deadline to opt in. Honestly did not care since I did not think I would ever come back to it to be honest.
When I logged in I was so lost though, so it's probably better it didn't carry over. Give me a fresh start and relearn how to even play!
You might be thinking of the transition from RS Classic to RS2! Players were given the option to migrate their characters then, but not from RS3 to OSRS.
You covered it pretty well. If you use runelite with the quest plugins it becomes quite simple to progress, and as you do quests you will find yourself unlocking content and levelling up skills for further quests.
Also don't be afraid of using the osrs wiki, it's the best wiki in gaming in my opinion. Just Google osrs thingtosearch and you will get easy to digest info