A compound comprising a synthetic ribonucleic acid comprising at least one modified nucleoside and coding for a telomerase reverse transcriptase for use in a method of treatment or prevention of (a) a disease involving short telomeres selected from the group consisting of metabolic syndrome, diabetes, diabetic ulcers, heart disease, cancer, vascular dementia, Alzheimer's disease, stroke, age-related macular degeneration, immunosenescence, bone marrow failure, gastrointestinal ulcers, cirrhosis, hernia, infection, chronic infection, mild or severe cognitive impairment, impaired mobility, osteoporosis, osteoarthritis, rheumatoid arthritis, age-related anxiety, balance disorders, tinnitus, Bell's palsy, cataracts, chronic obstructive pulmonary disease, corneal abrasion, coronary artery disease, peripheral artery disease, conjunctivitis, chalazion, dehydration, depression, emphysema, eye disease, failure to thrive, flu, generalized anxiety disorder, glaucoma, hearing loss, loss of sense of taste, loss of appetite, hip dislocation, memory loss, Parkinson's disease, spinal stenosis, urinary incontinence, and vertebral fracture; or (b) an age-related illness in an animal subject suffering from or being at risk of an age-related illness, wherein the age-related illness involves short telomeres; wherein telomerase reverse transcriptase is expressed transiently in a cell; and wherein at least one telomere is extended within the cell; and wherein the at least one modified nucleoside comprised in the synthetic ribonucleic acid confers reduced immunogenicity to the synthetic ribonucleic acid. (a) a disease involving short telomeres selected from the group consisting of metabolic syndrome, diabetes, diabetic ulcers, heart disease, cancer, vascular dementia, Alzheimer's disease, stroke, age-related macular degeneration, immunosenescence, bone marrow failure, gastrointestinal ulcers, cirrhosis, hernia, infection, chronic infection, mild or severe cognitive impairment, impaired mobility, osteoporosis, osteoarthritis, rheumatoid arthritis, age-related anxiety, balance disorders, tinnitus, Bell's palsy, cataracts, chronic obstructive pulmonary disease, corneal abrasion, coronary artery disease, peripheral artery disease, conjunctivitis, chalazion, dehydration, depression, emphysema, eye disease, failure to thrive, flu, generalized anxiety disorder, glaucoma, hearing loss, loss of sense of taste, loss of appetite, hip dislocation, memory loss, Parkinson's disease, spinal stenosis, urinary incontinence, and vertebral fracture; or (b) an age-related illness in an animal subject suffering from or being at risk of an age-related illness, wherein the age-related illness involves short telomeres; wherein telomerase reverse transcriptase is expressed transiently in a cell; and wherein at least one telomere is extended within the cell; and wherein the at least one modified nucleoside comprised in the synthetic ribonucleic acid confers reduced immunogenicity to the synthetic ribonucleic acid. wherein telomerase reverse transcriptase is expressed transiently in a cell; and wherein at least one telomere is extended within the cell; and wherein the at least one modified nucleoside comprised in the synthetic ribonucleic acid confers reduced immunogenicity to the synthetic ribonucleic acid.