NHS guidance says around the middle of the first year, but no earlier than 17 weeks. They also recommend to wait until the baby is showing signs of readiness, which they quote as:
- Sitting stably, with support if necessary, as this minimises the risk of choking - babies should never be fed solids in a reclined position (as used to be common).
- Loss of tongue thrust reflex, which is hard to know, but if you give them solids and they push them right out, it's probably too early.
- Being able to hold items and bring them to their mouth/having interest in putting items in their mouth by themselves. You'll see them doing this with their toys.
This is because we have good evidence that solid foods before 17 weeks is associated with long term digestive problems. The evidence for delaying from 17 to 26 weeks (6 months) is less clear but it is clear that there is no harm in waiting until 6 months, so it is recommended to do this because it eliminates all doubt. You also do not have to go very quickly at 6 months - this is a common myth (start at 4 months and go slowly with rice vs start at 6 months and be on 3 varied meals straight away) - but it's not necessary. You can choose a slow and steady approach whatever age you start, and you can choose a self-feeding based (BLW) approach from any time that the baby can sit up and hold food and get it to their own mouth (IME, from about 5-6 months)
Part of the way the recommendations are worded may be because, as you are finding, in some circles particularly in people who had their babies earlier than the 90s/00s there is a very big push towards early weaning. So if you tell people they can start at 17 weeks what you'll find is this cultural pressure interacts and people start at 12-16 weeks instead, which is quite risky. Aim for 6 months, and even if you get tempted to start early, it will probably be later than 17 weeks and therefore minimises the risk of harm.
The other cultural issue in the UK is that weaning is seen as something that should be progressed on quickly - if you get the various baby weaning guides from the food producers etc you will find that they all suggest progressing from first tastes to 3 meals a day in about 4-6 weeks, and the danger is if you go too quickly too early, you will push milk out of the baby's diet. Breastmilk or formula, according to NHS advice, should make up more than 50% of their calories for the majority of the first year - it's OK if food starts to take over by about 9-12 months, (and equally, it's also fine if your baby is slower and sticks to mostly milk for the full first year) but in the 80s when 3 month weaning was common, it used to be advised to swap to 50%+ food by 6 months. This might be where the idea it's a fast transition comes from. The name doesn't help as it suggests the idea is to swap milk for food - in reality the milk feeds continue, until 12 months if formula feeding (can continue with cow's milk if desired for comfort e.g. a bedtime feed) and into the second year and beyond if you want to for breastfeeding. The NHS is moving towards the term "Complementary feeding" to try and make this a bit clearer - solids are an addition to milk, at least until 12 months, after that they can transition to mostly food, but milk feeds may continue alongside. as long as it suits you both. You may also wish to ask your health visitor whether vitamin drops are required. First infant formula continues to be suitable for the whole 12 months and is more tightly regulated than follow on formula, so you might like to continue with this. If you want to change to follow on formula, you need to wait until the baby is at least 6 months and ideally wait until they are getting a good amount of solid foods, as it's intended to provide a complementary nutritional profile rather than a complete one.
It is true that in some other countries 4 months is given as the weaning age, but these countries have different cultural norms so the push for early weaning and fast weaning is less. For example, I live in Germany, and we are given a little chart that explains weaning as a 5 month process, adding one meal each month (starting with meat and potatoes!!) and then the fourth month is for changing from small puree meals to more typical family food and the fifth for adding snacks. So TBH I would not worry too much about what other countries say because you probably are not getting the whole picture, when people say things like "All the European countries start at 4 months and it's fine!"
If you want more info, your health visitor likely runs a free weaning course - ask her. First Steps Nutrition Trust is another free, not-for-profit resource aimed at parents of infants.
For paid content, Lucy Ruddle has an online starting solids course which runs monthly which I understand to be very good, if you like video content. And Amy Brown's book Why Starting Solids Matters has a lot of interesting info as well, if you prefer reading/audible.