Is it too late to start an online store?

Too late to open a general store, and not remotely too late to open a specific one. What has closed is the window where a broad catalogue and cheap advertising were enough.

What decides it

What replaced it favours the small operator: an audience that trusts you, a category you actually know, service a large retailer cannot afford to give. None of those existed as advantages when traffic was cheap enough to buy the difference.

The part that gets skipped

An idea is worth almost nothing on its own. What is scarce is somebody who will keep going after the first version does not work, which is every first version.

The best ideas usually come from a job somebody already had. You notice a problem because you lived inside it, not because you brainstormed.

A market that already spends money is easier than a market you have to convince exists. Competition is evidence, not a warning.

Where an app fits

If the idea is a shop, the sequence matters: sell first, prove people come back, then build the app. An app before the customers is a cost with nothing to do.