The NHS guidelines are 6 months. The signs that they are ready are that they can sit by themselves and can bring food to their mouths by themselves and swallow it.
As far as I'm aware the 4 months plus on these foods follow the old guidelines of weaning and they havent changed the packaging. Obviously they can make more money if they sell more.
We started weaning at 5.5 months as Sophie could sit very well unassisted and was picking food up and putting it in her mouth. We did BLW so let her guide us when she was ready. She was probably ready a 5 months really but we delayed it a few weeks as wanted to get a bit closer to 6 month mark.
Oh and just to add each to their own but baby rice and rusks aren't great nutritionally. We haven't used these at all but that's just my opinion
^^ WSS.
The NHS guidelines have remained unchanged for 10years, babies don't need anything but breastmilk or formula until 6months of age.
There is a change that happens within your babies gut BETWEEN 4-6months, meaning some babies digestive tract will be 'ready' sooner than others, but wean too early and you risk damaging the digestive tract which is very hard to correct.
Your LO watching you eat is NOT a definitive sign. Babies are naturally alert and inquisitive, if you started dancing round the room they'd watch, eating, they watch.
Sitting unassisted is a good indicator (and a necessity for BLW), they've lost their tongue thrust, food to mouth and swallow all which are more likely to occur nearer 6 months of age.
Rusk are unsuitable for under 6m due to the gluten (and they're full of shit) and baby rice is pretty nasty stuff and about as nutritionally balanced as cardboard so I would steer clear of those foods.
I began at 5.5m with homemade foods & finger foods, I think M was ready earlier but still waited until closer to 6m to make sure she was truly ready.