That's yellow. Somewhere around 30 years old I should think. There may be a date on the commissioning label. No need to replace it unless it starts leaking, if it is big enough for your needs.
Newer ones have better insulation, and, more importantly, they have a bigger heating coil inside so the boiler can reheat them faster. A modern boiler can reheat a modern cylinder in 20-30 minutes, so you can have one bath followed by another.
The immersion heater goes in at the top, which is not a problem, except that, as warm water rises, and it does not reach the bottom of the cylinder, it can't give 140 litres. It will probably have about enough for one bath, depending how wide the cylinder is (might be 18" or 24")
If you ever buy a new cylinder, it's preferable to have a good big one, with an immersion heater at the top and another at the bottom, so you use according to the amount of hot water you need. It is also very strongly recommended to have a modern unvented cylinder, which runs at mains pressure, so you need no pump, and no loft tank. You will probably have to replumb the water supply pipe, which is not as much work as you might think, as the old one is probably only half-inch and can't deliver water as fast as you want. This will give truly unsurpassed hot water.
A new cylinder will work with your old boiler, or a new one. Again, no need to change your boiler if the old one has enough power to heat your house and HW, unless it breaks down and parts are unobtainable, even to someone who is actually trying to find them*
*this excludes some companies that would rather sell you a new boiler than mend the old one