How much can I expect to improve at 34 years of age?

This is my first post here. I am 34 years old from Sweden. I started training about 1 year ago. Before was only recreational swimming once per year or so, which is equivalent to nothing. I swim between 2-3 km per day 5-6 days per week. I usually swim around 15 minutes per km. I believe I started out on 17 minutes or so per km. I wonder if this is a normal learning curve, and how much can I expect to improve in the coming years with current training. Lately I have been practising a lot of technique, which dropped me from around 16 minutes to 14:40 as a best in 1 km in just a matter of 1 month. I attriute most of that to imroving my technique. I only swim myself, but I try to read up on technique in books such as TI, and oter books. I suppose it's normal to reach a plateau once in a while (which I am in right now). Even though I swim 15 minutes per km now, it feels a lot easier than it did before, and I am less tired. My goal by next summer is to reach 14 minutes. However it is difficult to swim any good times now, when there are a lot more people in the pool in the fall, winter, and spring. In the summer I basically have the pool for myself, and then the water is all calm, and I can practise anything I want. Now it is even difficult to swim 250 meters in one go. 1 km feels like a luxury these days. Maybe it's because I don't swim 1 km every day continously that I don't improve. Many times I do 2 kms of 250s, 100s, 50s, etc. Sometime I try to swim during my lunch break, when the pool is nearly empty. So my question is if anyone else has been in a similar situation, and how much have you improved and how long did it take. I know people will say it's depending on the person, training, physical shape, etc. Btw I am 6"3, weighing 185 lbs, and in decent shape, and highly motivated. Thanks for a great forum.