I've noticed that since the implementation of pagination the 'age' of a thread is calculated on each page independently. This means that if a threads OP and first page only has posts that are more than 2 months old it will display the 'post closed as over 2 months since last post' message, however if you navigate to a newer/the newest page there may be posts that are within the past 2 months and the reply box will happily reappear. This can be/has been confusing for people looking at large threads for the first time and therefore being shown page 1 intitially.