Should the existing base be replaced if it's condition is fine?

The base (maker uncertain) is 20 years old (super-king / split longways) with a 5" cushion top. It shows no signs of wear in that it's level and there's no undue compression, even when sitting on the edge. Does it follow automatically that a base of this age needs replacing, or is there a way of ascertaining its condition?
The mattress does need replacing - John Lewis Natural Collection Leckford Linen 5000 in Firm; less than a year old but so hard that it's unbearable.
Many thanks.