Should I consider replacing bottom bracket?
I would have though that after  18 years and 45000km the bottom bracket would be worn out. I recently learned that bearing wear is heavily dependent on load (source: GCN Everything You Need to Know About Bearings) so if you are a light rider and are not putting out lots of power I guess the bottom bracket bearings could last that long.

If you are not feeling any play in the bottom bracket bearings and they turn smoothly that's further evidence the bearings are OK. If you can you should pull the cranks off and turn the axle with your fingers. You may find you feel drag or 'notchiness' that the mass of the cranks is disguising.  

You very likely have a cartridge type bottom bracket bearing unit, these are not particularly expensive and are available from a number of vendors so you may as well replace it. I'd check the chain with a wear gauge and consider replacing it, and look at the chainrings and sprockets as well.