Your first sentence is ambiguous; it may represent either of two different uses of would.
If Google is translating your grammar book approximately correctly (which is not something I put a great deal of confidence in!), it seems to offer this sentence as an example of both a) would, used as the past tense of will, and b) would, used to ‘speculate about the past’. These two are not entirely compatible.
Let's begin with a) would, used as the past tense of will. In this case, your sentence represents sentence A1 ‘backshifted’ into the past as A2. I’ve added clauses on the front and back to clarify the time references:
A1. [She says today that] he will be about twenty when he crosses ... [next year].
A2. [She said ten years ago that] he would be about twenty when he crossed ... [the next year].
The ‘speculative’ use of b) would, is different: it represents its clause as uncertain, based on inferences or on counterfactual conditions.
B1. [If he had not died,] he would be about twenty ....
B2. [If her math is right,] he would be about twenty ...
But note that there is a significant difference between B1 and B2. B1 requires he would be, because it is counterfactual. B2, however, may just as correctly be expressed as he is—or even he will be.
B3. [If her math is right,] he will be about twenty ...
(Will is tolerated because where inference is involved, will no longer necessarily bears a ‘future’ implication. Its sense is still sequential; but the sequence is logical rather than temporal, as when we say “A follows logically from B”.)
Accordingly, backshifting this sort of speculative clause can be effected with the same form as in A2:
B4. [If her math was right,] he would be about twenty ...
However, this represents a past speculation about what were then present (or future) contingencies. If you want to represent a present speculation about past contingencies, you need a different construction:
C1. [If her math is right,] he was about twenty ...
C2. [If her math is right,] he would have been about twenty ... [preserving would]
So whether the construction is awkward depends on what you mean.