Soft eggs can be a sign that either your hen is young and doesn’t quite have the ‘knack’ of laying yet OR it can be indicative of the need for more calcium in their diet. In that instance, you’d want to add some crushed oyster shell or similar to their feed.
Any time between 20-25 weeks old start looking for eggs from your hens. Don’t start feeding them layer feed until after you’ve seen an egg. Once one does it, the rest generally follow the leader.
Generally older chickens slow down before they stop all together and even then they may still lay an egg every few weeks before they pass away. For most breeds, you’re looking at five to seven years of active egg laying before they start to slow down.