That’s probably not a very reliable proxy for age estimation though (for reasons alluded to by sampo, and simply because telomere length varies across individuals regardless of age). It’s probably much easier and more reliable to measure the accumulation of single-base mutations (SNPs): DNA repair is faulty and such mutations accumulate with age. By comparing the genetic material of several cells across a sample this can be estimated.
However, telomeres are repetitive code, and not amenable to standard PCR and sequencing methods (as far as I know). I don't know if the amounts of DNA or chromosomes typical in forensic samples are enough for the techniques used to measure telomere length. Well maybe they are, if the Danish group is working on this.
Well I think that if the length of the fragment is larger than the repeated fragment, you can get a good idea of the number of repetitions.
This is not a case like CNV (copy number variation) where a whole gene can appear a number of times, so I think this is somewhat possible by PCR (though probably not very precise).
The problem in finding repetitions using PCR is as follows: In order to perform PCR you splice the DNA in chunks randomly, the length of the chunks depends on the particular machine but it's of the order of 100-1000 bases. Then you need algorithmically find superpositions to reconstruct the original sequence.
If the repeated sequence is longer than the chunk you have no hope of knowing how many times it repeats, if on the other hand it's much shorter (I think it's 6 bases for telomeres) you can see a good number of repetitions per chunk.
If you want to know something more detailed about sequencing and CNV I can send you a couple of sections from my PhD dissertation.
https://en.wikipedia.org/wiki/Telomere#Shortening