"I would have had children, if I were not so old."
Translation:Jeg ville have haft børn, hvis jeg ikke var så gammel.
The English tenses do not seem to match the pattern of past conditionals; see for example https://www.englishpage.com/conditional/pastconditional.html .
The second part of the sentence is a subordinate clause. In subordinate clauses the adverb (in this case ikke) appears before the conjugated verb instead of after. Otherwise it works like any other sentence.
Your sentence would need to be "Jeg ville have haft børn, hvis jeg var yngre."